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Basics of
Epidemiology

Book list:
1
...
Park’s Textbook of Preventive and Social Medicine
By K Park
3
...
Merrill
4
...
A
...
Introduction to Epidemiology
Epidemiology: The term epidemiology originates from the Greek terms epi (upon), demos (people) and logos (study)
that literally means the study of what is upon the people
...

Major components of the definition:
1
...


3
...

5
...
For example, malaria affects many people in Ethiopia but lung cancer is rare
...
Even though lung cancer is more killer, epidemiology
gives more emphasis to malaria since it affects many people
...
Epidemiology is concerned with
the frequency (occurrence) of diseases and other health related conditions
...

Distribution: Distribution refers to the geographical distribution of diseases, the distribution in time and
distribution by types of persons affected
...

Health related conditions: Epidemiology is concerned not only with diseases but also with other health related
conditions because everything around us and what we do also affects our health
...
These may be injuries, births, health related
behaviors like smoking, unemployment, poverty etc
...
Morris, a prominent British epidemiologist
are as follows:
1
...

2
...

3
...

• Effectiveness: Whether it works in the ‘real world’ or not and how it works
...
Individual risks and chances: It includes –
• Actual risks
• Health hazards/risk appraisal
5
...

6
...

7
...

8
...

Some of them are discussed below:
1
...
For
example, new diseases appear (AIDS, SARS), some are eradicated (Small pox, polio in the Americas) and still
others are reduced to very low levels (mastoiditis, rheumatic fever)
...
This is one of
the basic responsibilities of any health system
...

2
...

• How healthy is the population?
• Is its health getting better or worse?
• Are some areas or subgroups much healthier than others?
• How can we qualify these differences?
• What is the impact of ill health on society? How can we measure this?
• What are the population’s health needs?
• What risks does it face?
• What explains the differences in health?
Mortality (death) is an available indicator of the health of a community but this is a downstream measure
of population health
...

Epidemiology uses a number of more upstream indicators to assess the health of a population beyond
mortality and morbidity such as self-related, psychological well-being, activity limitation, life expectancy
and potential years of life lost (PYLL)
...


3
...

• What percentage of the contacts of a case of multi-drug resistant tuberculosis have been located and
assessed?
• How many of the eligible children in a community are adequately immunized by school entry?
• How many are immunized on time, according to the recommended schedule?
• What proportion of restaurant workers has received adequate training in food handling?
• How often are supermarket meet coolers subjected to routine inspections?
• Are all women who become pregnant receiving antenatal care in the first trimester of their pregnancy?

4
...

• What are the chances one will be alive five years from now?
• Is one going to start that exercise program today?
• Isn’t it time to make an appointment to have that Placental Alkaline Phosphatase (PAP) smear
repeated?
• One really should have an influenza shot again this year
...


Principles of epidemiology:
1
...

3
...

5
...

Identifying risk factors associated with the problem
...

Implementing interventions to improve the health of the population and
Monitoring those interventions to assess their effectiveness
...


2
...

4
...


Population: The main focus of epidemiology is on the effect of disease on the population rather than
individuals
...
If an individual
develops lung cancer, it is likely that, he/she will die
...

Frequency: This shows that epidemiology is mainly a quantitative science
...
Frequency of diseases is measured
by morbidity and mortality rates
...

Determinants: Determinants are factors which determine whether or not a person will get a disease
...
Health related conditions are
conditions which directly or indirectly affect or influence health
...


Modern concept

5

2
...

Components of epidemiological triad: It has three components:
•
•
•

An agent
A susceptible host and
An environment that brings the host and agent together
...
When all three in harmony,
health is ensured but maladjustment in their relationship leads to disease
...
Examples include
Host: Host refers to the organism e
...
, human capable of being infected by a specific agent
...
These factors include socioeconomic status, lifestyle, behaviors and psychological characteristics
...
Environmental factors are extrinsic factors that affect
the agent and the opportunity for exposure
...
g
...
g
...
g
...


Host

Agent

Environment

Agent: The agent has been defined as an element, a substance or a force either animate or inanimate, the presence or
absence of which may, following effective contact with the susceptible human host and under proper environmental
conditions, serve as a stimulus to initiate or perpetuate a disease process
...

2
...


4
...
Examples include viruses, fungi, bacteria, protozoa,
metazoan
...
Any excess or
deficiency of the intake of nutritive elements may result in nutritional disorders
...
Examples are urea (uraemea), serum bilirubin (jaundice), ketones
(ketosis), uric acid (gout), calcium carbonate (kidney stone)
...
g
...

Mechanical agents: Exposure to chronic friction and other mechanical forces may result in crushing, tearing,
sprains, dislocations or even death
...

6
...


Social agents: These include poverty, smoking, abuse of drugs and alcohol, unhealthy life styles, social
isolations, material deprivation
...

Psychological agents: Work stress is an example of psychological agent
...

Host: Hosts are organisms, which give shelter and nourishment to the parasite
...
Definitive host
2
...
Accidental host
4
...


a
...
Non-permissive host
Paratenic host (a carrier or transporter host)
Reservoir host

They are discussed below:

1
...


3
...

5
...
In the majority of human parasite infections, man is the definitive host, such
as Ancylostoma duodenale, Ascaris lumbriciodes etc
...
In some cases, larval
developments are completed in two different intermediate hosts which are being referred to as first and second
intermediate hosts respectively, e
...

• Man is the intermediate host for malaria parasites
...

Accidental hosts: Accidental hosts are those in which the parasite does not normally develop but when
occasionally infections occur accidentally, the parasite is able to complete its lifecycle
...
Permissive hosts: Permissive hosts are those hosts where the parasite can complete its lifecycle
...

b
...
These are the dead end hosts e
...
man is
the accidental non-permissive host for Echinococcus granulosus
...
g
...

Reservoir host: A reservoir host is a vertebrate animal species that harbors a particular pathogen and acts as
a long-term source of infection for other vertebrates or vectors
...
g
...


Environment: The environment is the sum total of all external conditions and influences that affect the life and
development of an organism
...
Environment can be categorized as:
•
•
•

Biological environment: This includes infectious agents of disease, reservoir of infection, vectors that transmit
disease, plants and animals etc
...

Physical environment: Heat, light, air, water, radiation, gravity, chemical agents etc
...


The various types of environments play the critical role to live, grow up and multiplication of both agents and hosts
...


7

As a contrast, a good environment may promote human health by encouraging healthy behaviors and preventing
growth and reproduction of vector of diseases
...
In addition, there are factors other than communicable diseases that can influence human health
outcomes
...
That’s
why environment plays the vital role in epidemiological triad
...
Dynamics of Disease Transmission
Infectivity: The ability of an infectious agent to invade and multiply (to produce successful infection) in the host is
called infectivity
...
An infection does not always cause a disease
...
Gastrointestinal tract is an exterior surface for infestation of intestinal
parasite, e
...
warm infestation
...
The infectiousness depends on –

•
•
•
•

Mode of transmission
Portal of entry and portal of exit
Agent characteristics of ability to survive away from host
Infectivity

Infectious disease: A clinically manifest disease of man or animal resulting from an infection is called an infectious
disease
...
g
...

Infective dose: Infective dose of an agent is the amount required to cause infection in susceptible host
...

Importance of incubation period in epidemiology:

1
...

3
...

5
...

Useful in determining the period of surveillance or quarantine
...

Helps to identify the point source epidemics
...


Generation time: The time period when the host is capable of transmitting the disease to susceptible individual before
the development of clinical feature of the disease in the host is called generation time
...

Communicable disease: A disease which can get transmitted from one host to reservoir to another through transference
of a sufficient quantity of the causative agent is called a communicable disease, e
...
tetanus, malaria, enteric fever,
tuberculosis etc
...
Human, animal and environment can serve as reservoir
...
Homologous
• Cases
• Carriers
B
...
Frank case
• Recognizable
• Typical clinical manifestations
b
...

2
...


Human reservoirs
Animal (non-human) reservoirs and
Environmental reservoirs

They are discussed below:
1
...


3
...
Poliomyelitis and small pox exist exclusively within a human reservoir
...
For example, the bacterium Vibrio cholerae, which causes cholera in humans, has
natural reservoirs in copepods, zooplankton and shellfish
...

Environmental reservoirs: Environmental reservoirs include living and non-living reservoirs that harbor
infectious pathogens outside the bodies of animals
...
Pathogens found in these reservoirs are sometimes free-living
...


Factors affecting disease transmission:
The overall health of the host has a clear influence on the disease process
...

Occupation: More infections are seen in health care workers
...
War / Stress
Climate: It appears that there is a greater incidence of respiratory infections in colder climates
...

Genetic Predisposition

10

Herd immunity: Herd immunity (also known as community immunity) is the protection of an individual from infection
by virtue of other members of the population (the herd) being incapable of transmissting the virus to that individual
...

1
...


Natural infection: For example, if a viral disease, such as measles occurred in approximately 90% of a group
and if those who recovered from the disease had sufficient immunity to prevent them from becoming infected
and serving as a source of virus for others, then the remaining 10% of the group are protected by herd immunity
...
Live, attenuated polio provides herd immunity because –
a
...

b
...


Carrier: A carrier contains, spreads or harbors an infectious organism
...
For example,
rodents or coyotes are often carrier of Bubonic Plague
...

Classification:
A
...
Active carrier: Individual who has been exposed to and harbors a disease causing organism (pathogen) and
who has done so for some time, even though the person may have recovered from the disease
...
Convalescent carrier: Individual who harbors a pathogen and who, although in the recovery phase of the
course of the disease, is still infectious
...
Healthy carrier (also called passive carriers): Individual who has been exposed to and harbors a pathogen
but has not become ill or shown any of the symptoms of the disease
...

4
...

5
...

6
...
g
...

B
...
Oral carrier
2
...
Intestinal carrier
4
...
Blood or tissue carrier
C
...
Fecal carrier
2
...

2
...

They readily infect the susceptible individuals over a wider areas and longer period of time under favorable
conditions
...
Disease transmission occurs when the pathogen leaves the reservoir (e
...
, food, water, feces) through a portal
of exit (e
...
, nose, mouth, rectum, urinary tract, blood, other bodily fluids) and is spread by one of several modes of
transmission
...
g
...


11

Fig: Mode of disease transmission
...
Disease
prevention means to interrupt or slow the progression of disease
...

There are 3 levels of prevention:
1
...
Secondary prevention and
3
...


2
...
Primary prevention keeps the disease processes from becoming established by
eliminating causes of disease or increasing resistance to disease
...
Health promotion: It consists of general non-specific interventions that enhance health and the
body’s ability to resist disease
...

b
...
Examples can be provision of safe and adequate water, proper excreta disposal
and vector control
...
Prevention of disease: It is the prevention of disease development after the individual has become
exposed to the disease causing factors
...

Immunization acts after exposure has taken place
...

If we take measles vaccine, it will not prevent the virus from entering to the body but it prevents
the development of infection or disease
...
Secondary prevention can be achieved through detecting people who
already have the disease as early as possible and treat them
...

Examples:
• Prevention of blindness from trachoma
...


Early detection and treatment of breast cancer to prevent its progression to the invasive stage, which
is the severe form of the disease
...
Tertiary prevention is needed in some disease because primary and secondary preventions have
failed, and in others because primary and secondary prevention are not effective
...

• To limit the physical, psychological, social and financial impact of disability, thereby improving the
quality of life
...

Example: When a person becomes blind due to vitamin A deficiency, tertiary prevention (rehabilitation)
can help the blind or partly blind person learn to do gainful work and be economically self-supporting
...

Components of the infectious process
The infectious process of a specific disease can be described by the following components, which constitute of the chain
of disease transmission
...
The agent
2
...
Its portal of exits
4
...
Its portal of entry
6
...

Reservoirs: A reservoir is an organism or habitat, in which an infectious agent normally lives, transforms,
develops and/or multiplies
...
Some diseases with human reservoirs are AIDS and sexually transmitted infections,
measles, typhoid etc
...
Possible portals of exit include
all body secretions and discharges: Mucous, saliva, tears, breast milk, vaginal and cervical discharges,
excretions (feces and urine), blood and tissues
...

Mode of transmission: Modes of transmission include the various mechanisms by which agents are conveyed
to other susceptible hosts
...

•

Direct transmission:
a
...

Example: HIV/AIDS, STIs, rabies
...


•

Direct projection: It is transmission by projection by saliva droplets during coughing, sneezing,
singing, spitting or talking
...

c
...
Examples are
syphilis, HIV/AIDS
...

a
...


13

b
...

Vectors may be biological or mechanical
...

Biological vector: A vector is called biological if the agent multiples in the vector before
transmission
...

ii
...
The vector simply carries the agent by its body parts (leg, proboscis etc
...
Example: Flies are mechanical vectors for the transmission of trachoma
...
Airborne: It may occur by dust or droplet nuclei (direct residue or aerosols)
...
When pulmonary tuberculosis patients cough, they emit many aerosols which
consists the agents of tuberculosis
...
These
droplet nuclei will remain suspended in the air for some time
...

Portal of entry: It is the site where an infectious agent enters a susceptible host
...

• Conjunctiva is the portal of entry for trachoma
...

Susceptible human host: The susceptible human host is the final link in the infectious process
...
Host resistance at the
community level is called herd immunity
...
Example: If 90% of the children are vaccinated for measles, the remaining 10% of the
children who are not vaccinated might not become infected with measles because most of the children (90%)
are vaccinated
...


14

4
...
Rate
2
...
Proportion
They are discussed below:
1
...
Rate is a special form of
proportion that includes time
...
Accurate count of all events of interest that occur in a defined population during a specified period
is essential for the calculation of rate
...


Example: The number of newly diagnosed pneumonia cases in 1999 per 1,000 under five children
...


Ratio: A ratio quantifies the magnitude of one occurrence or condition to another
...

𝑦
Example:
•
•

3
...

The ratio of male malaria patients to female malaria patients
...
It is a specific type of ratio in which the numerator is included in the denominator and the result is
expressed as a percentage
...


Measures of mortality
Mortality rates and ratios measure the occurrence of deaths in a population using different ways
...
This is done because population size fluctuates over time due to births, deaths and migration
...
Crude death rate
2
...
Infant mortality rate
4
...
Still birth rate
6
...
Neonatal mortality rate
8
...
Case fatality rate
10
...
Proportionate mortality ratio
12
...
It reflects the risk of death in that community or country
...
6 per 1000 population
...

CDR =

𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑟𝑒𝑝𝑜𝑟𝑡𝑒𝑑 𝑑𝑢𝑟𝑖𝑛𝑔 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙
𝑒𝑠𝑡𝑖𝑚𝑎𝑡𝑒𝑑 𝑚𝑖𝑑 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛

× 100

Age specific death rate:
Age specific death rate =

𝑛𝑜
...

Infant mortality rate: Infant mortality rate reflects the health of the community in which the child is being brought up
...
Malnutrition is also one of the killer of infants in developing countries
...
That means out of 1000 live births about 97 die
before they celebrate their first birth day
...
𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑢𝑛𝑑𝑒𝑟 1 𝑦𝑒𝑎𝑟 𝑜𝑓 𝑎𝑔𝑒 𝑑𝑢𝑟𝑖𝑛𝑔 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒
𝑛𝑜
...

Still birth rate:

Perinatal mortality rate:
Neonatal mortality rate:
NMR =

𝑛𝑜
...
𝑜𝑓 𝑙𝑖𝑣𝑒 𝑏𝑖𝑟𝑡ℎ𝑠 𝑟𝑒𝑝𝑜𝑟𝑡𝑒𝑑 𝑑𝑢𝑟𝑖𝑛𝑔 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑡𝑖𝑚𝑒

× 1,000

Example: In 1996 there were a total of 5000 live births in Zone X
...

So the neonatal mortality rate
NMR

200
5000

= × 1000

= 40
That means out of 1000 live births in 1996, 40 of them died before 28 days after birth
...
𝑜𝑓 𝑝𝑟𝑒𝑔𝑛𝑎𝑛𝑐𝑦 𝑎𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑒𝑑 𝑑𝑒𝑎𝑡ℎ𝑠 𝑜𝑓 𝑚𝑜𝑡ℎ𝑟𝑠 𝑖𝑛 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒
𝑛𝑜
...
The
maternal mortality rate in Ethiopia is estimated to be 871 per 100,000 live births
...


16

Case fatality rate: Case fatality rate represents the probability of deaths among diagnosed cases or the killing power of
a disease
...
𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑓𝑟𝑜𝑚 𝑎 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑑𝑖𝑠𝑒𝑎𝑠𝑒 𝑑𝑢𝑟𝑖𝑛𝑔 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒
𝑛𝑜
...
Out of the 1000 patients 100 died in the same
year
...

Under 5 mortality rate:
Under 5 mortality rate =

𝑛𝑜
...
In the same year 200
children under 5 years of age died
...

Proportionate mortality ratio:
PMR =

𝑛𝑜
...
𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑓𝑟𝑜𝑚 𝑎𝑙𝑙 𝑐𝑎𝑢𝑠𝑒𝑠 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑡𝑖𝑚𝑒

× 100

The proportionate mortality ratio asks the question: What proportion of deaths are due to a certain cause? For example,
when we say the proportionate mortality ration for HIV/AIDS is 30%, this means out of 100 total (of all) deaths 30 of
them died from HIV/AIDS
...
𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑖𝑛 𝑎 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑠𝑒𝑥 𝑑𝑢𝑟𝑖𝑛𝑔 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒
𝑒𝑠𝑡𝑖𝑚𝑎𝑡𝑒𝑑 𝑚𝑖𝑑 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑠𝑎𝑚𝑒 𝑠𝑒𝑥

× 1000

Example: The average total population of X in 1996 was 6000 (2500 male and 3500 female)
...
So
Sex specific mortality rate =

100
3500

× 1000

= 29 per 1000 female population
That means out of 1000 female population living in X, 29 females died in 1996
...
Measures of morbidity include incidence rate,
period prevalence rate and point prevalence rate
...

Incidence rate =

𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑛𝑒𝑤 𝑐𝑎𝑠𝑒𝑠 𝑜𝑓 𝑎 𝑑𝑖𝑠𝑒𝑎𝑠𝑒 𝑜𝑣𝑒𝑟 𝑎 𝑝𝑒𝑟𝑖𝑜𝑑 𝑜𝑓 𝑡𝑖𝑚𝑒
𝑡𝑜𝑡𝑎𝑙 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑑𝑢𝑟𝑖𝑛𝑔 𝑡ℎ𝑒 𝑔𝑖𝑣𝑒𝑛 𝑝𝑒𝑟𝑖𝑜𝑑 𝑜𝑓 𝑡𝑖𝑚𝑒

× k
...
Because incidence is a measure of new events
i
...
, transition from a non-diseased to a diseased state, incidence is a measure of risk
...
Hence, total population
can be used as a denominator
...
For incidence to be a measure
of risk, we must specify a period of time and we must know that all the individuals in the group represented by the
denominator have been followed up for that entire period
...
Incidence rates can be used to make statements

18

about the risk of disease
...

Uses of incidence rate: Incidence rate is important as a fundamental tool for etiologic studies of diseases since it is a
direct measure of risk
...

Prevalence rate: Prevalence rate measures the number of people in a population who have a disease at a given time
...
The major type of prevalence is point prevalence rate
...
Point prevalence rate can be determined by conducting cross-sectional study
...


Example: One health extension worker conducted a survey in one of the nearby elementary schools in Hidar 10, 1996
to know the prevalence of trachoma in that school
...
The health
extension worker examined all the 200 students for trachoma
...
So
Point prevalence rate =

100
200

× 100

= 50 trachoma patients per 100 students on Haidar, 10, 1996
...

Uses of prevalence rate:
•
•

Planning health facilities and human resource
...


19

5
...

Ecological, cross sectional, case control and cohort studies are analytical type studies
...
In an experiment, by contrast, the researcher
intervenes to change something e
...
, gives some patients a drug and then observes what happens
...

Examples of observational studies:
1
...

3
...

A researcher who joins a biker gang to study their lifestyle (as long as the researcher does not try to change
their behavior, it’s an observational study)
...
(one can intervene to
take the blood but doesn’t try to change the blood alcohol level; it’s just a measurement)
...

2
...


20

3
...

5
...


Can be a case report, case series, cross sectional or correlational studies
...

Tools include person, time and place dimensions of a disease
...


Uses of descriptive studies:
A
...
Gives a general guidance as to possible causes or determinants of diseases
...
Helps to determine more frequent occurrence of a particular disease in an occupational group
...
By this study, certain attributes or exposures are more commonly found amongst people who have
the disease or who do not have the disease
...
Clinical:
1
...

2
...
g
...

C
...
States the magnitude of health problems in various segments of the community
...
Monitors changes in disease incidence
...

2
...

4
...

6
...

Answer why and how questions
...

Enable risk assessment of diseases among populations
...

Study incidence rates and prevalence of diseases in populations
...
In this type of
study information is collected not on individuals but on groups of people
...

Example:
•

•

Investigators wished to study the hypothesis that the risk of melanoma skin cancer was related to the amount
of exposure to ultra-violet rays
...

Cholera study: The stud by John Snow regarding a cholera outbreak in London is considered the first
ecological study to solve a health issue
...
He had the pump handle removed in 1854 and the people stopped
dying there
...
Given that an association is demonstrated in
ecologic studies, and even if information is available on potential confounding factors, it does not guarantee that the
relationship holds at the level of the individual
...

Advantages:
1
...


Generally inexpensive
...


21

3
...


The outcome of such a study will be to conclude only that the study either supports or does not support a
hypothesis about a relationship between exposure and disease
...


Disadvantages:
1
...

3
...


Measures of exposure are only a proxy based on the average in the population
...

Potential for systematic differences between areas in recording disease frequency
...

Potential for systematic differences between areas in the measurement of exposures
...


Cross sectional studies: The study in which the presence or absence of disease or other health related variables are
determined in each member of the study population or in a representative sample at one particular time is called cross
sectional study
...
In some
instances information may be collected about disease over a period of time, e
...
, the past month or year
...

Examples:
•
•
•
•

Community survey
Prevalence of stroke
Prevalence of diabetes mellitus
Prevalence of heart disease

Steps of cross-sectional study:
1
...


3
...

5
...
For example, a study of the prevalence of diabetes
among women aged 40-60 years in town A should comprise a random sample of all women aged 40-60 years
in that town
...

Sample size calculation: The sample size should be sufficiently large enough to estimate the prevalence of the
conditions of interest with adequate precision
...

Data collection: As data on the exposures and outcomes are collected simultaneously, specific inclusion and
exclusion criteria should be established at the design stage, to ensure that those with the outcome are currently
identified
...
Routine data sources may also be used
...

Calculation of prevalence rate and odds ratio
...

2
...


It is relatively inexpensive
...

As the information is collected about disease state and exposures currently, the problem of recalling past events
is less than if subjects were asked about exposures about exposures and disease state in the medium or distant
past
...

2
...

4
...


There is lack of information on temporality
...

Confounders may be unequally distributed
...

Group sizes may be unequal
...


Establishes association at most, not causality
...

2
...

4
...

6
...


Cross sectional studies are used to estimate the prevalence of disease or the prevalence of exposure to risk
factors or both
...

It measures prevalence, not incidence of disease
...

It is useful in studying disease etiology
...

It is used to describe distribution pattern of the disease in population
...
It can assess
only prevalence of disease or other health events and so it is also called prevalence study
...
Exposure and
outcomes are assessed simultaneously
...


Design and analysis of a cross sectional study:
Disease

No disease

Exposed

a

b

Not exposed

c

d

23

Prevalence of exposure in disease and no disease:

𝑎
𝑎+𝑐

Prevalence of disease in exposed and not exposed:

𝑎
𝑎+𝑏

vs

𝑏
𝑏+𝑑

vs

𝑐
𝑐+𝑑

Case control study: This type of study investigates relationships between potential risk factors and disease
...
The cases
and controls are then compared with respect to their exposure to risk factors
...
In situations where information cannot be obtained from the cases (e
...
, very severe
illness or death), it may be appropriate to identify a proxy respondent for the case
...
Subjects will be required to recall past events (exposures) and there will be an issue of firstly whether they will
be able to recall such exposures accurately and secondly whether recall is similar in cases and controls
...
A case-control study
was conducted to investigate if exposure to zinc oxide is a more effective skin cancer prevention measure
...

This study would be retrospective in that the former lifeguards would be asked to recall which type of sunscreen they
used on their face and approximately how often
...

Case control study has three distinct features:
1
...

3
...

The study proceeds backwards from case to effect
...


Case selection:
Sources for identifying case patients:
•
•
•
•
•
•

Medical records
Laboratory results
Surveillance systems
Registers
Mass screening programs
Case-patients identify other persons who have similar illness
...
g
...

Relatives, friends or neighbors of cases
...

Matching may be of two types:
•
•

Group matching and
Individual matching

Problems with matching:
1
...


Practical problem with matching: If an attempt is made to match according to too many characteristics, it may
prove difficult or impossible to identify an appropriate control
...


Case-control studies have four main steps:
1
...

3
...


The study begins by enrolling people who already have a certain disease or outcome
...
They should not be selected because of an exposure status
...

Finally, an odds ratio is calculated
...

2
...

4
...


Good for studying rare conditions or diseases
...

It lets one simultaneously to look at multiple risk factors
...

Can answer questions that could not be answered through other study designs
...

2
...


Retrospective studies have more problems with data quality because they rely on memory and people with a
condition will be more motivated to recall risk factors (also called recall bias)
...

It can be difficult to find a suitable control group
...
In
Roman times a cohort was a body of 30-600 infantry
...
g
...
Thus a group of people who were born on a day or in a
particular period, say 1948, form a birth cohort
...
The aim is to determine whether initial exposure status

25

influences risk of subsequent disease
...

Examples:
Example

Exposure

Outcome

Smoking causes lung cancer

Smoking

Lung cancer

V
...
cholerae

Diarrhea

Salmonella causes diarrhea

Salmonella

Diarrhea

Rota virus causes diarrhea

Rota virus

Diarrhea

Other examples:
•
•

The Framingham study
Incidence of breast cancer and progesterone deficiency

Features of cohort studies:
1
...

3
...

The study groups are observed over a period of time to determine the frequency of disease among them
...


Indicators of cohort study:
1
...

3
...


When there is good evidence of exposure and disease
...
g
...

When follow up is easy, cohort is stable, co-operative and easily accessible
...


Types:
1
...
Framingham study is an example of prospective cohort study
...


Retrospective cohort study: A retrospective cohort study is one in which the outcomes have all occurred
before the start of investigation
...
It is known with the name of Historical cohort and Noncurrent
cohort
...

Now we find that an old roster of elementary schoolchildren from 1998 is available in our community and that
they had been surveyed regarding their smoking habits in 1998
...


26

Time

Study begins here
Fig: Retrospective study
Steps of cohort study:
1
...

3
...

5
...
Selection of study subjects: The usual procedure is to locate or identify the cohort, which may be a total
population in an area or sample therefore, cohort can be:
CEBO MD
• Community cohort of specific age and sex
• Exposure cohort, e
...
, radiologists, smokers, users of oral contraceptives
• Birth cohort, e
...
, school entrants
• Occupational cohort, e
...
, miners, military personnel
• Marriage cohort
• Diagnosed or treated cohort, e
...
, cases treated with radiotherapy, surgery, hormonal treatment
...
Obtaining data on exposure: Data can be obtained by the following means:
• From cohort members: Personal interview, mailed questionnaire
• Review of records: Certain kinds of information like dose of radiation, kinds of surgery received can
only be obtained from medical records
...

• Environmental survey of location where cohort lives
...
Comparison groups: Comparison groups may be• Internal comparison group: Single cohort enters the study and its members on the basis of
information obtained, can be classified into several comparison according to degree of exposure
...
For example, if all workers in the factory had some degree of exposure,
we would need to select a comparison group from another population, possibly another type of
factory
...

4
...

Procedure may be-

5
...

Analysis: Data analyzed in terms of• Incidence rate of outcome among exposed and non-exposed
• Estimation of risk
...

2
...

4
...


It is ethically safe
...

Can establish timing and directionality of events
...

Administratively easier and cheaper than some other studies
...

2
...

4
...


Controls may be difficult to identify
...

Blinding is difficult
...

For rare diseases, large sample sizes or long follow-up is necessary
...

Types of trials:
Trial

Controlled

Randomised

Blinded

Not controlled

Not randomised

Not blinded

Randomized controlled trials
It is a type of experimental study where subjects in the study population are randomly allocated to intervention and
control groups and the results are assessed by comparing outcomes
...
e
...
If the initial selection and randomization is done
properly, the control and treatment groups will be comparable at the start of the investigation; any difference between
groups are chance occurrences unaffected by the conscious or unconscious biases of the investigators
...
Selection of participants:

2
...


4
...


6
...
According to how the participants are exposed to the interventions, randomized controlled trials (RCTs)
can be classified as –
1
...
Crossover trials
3
...
According to whether the investigators and participants know which intervention is being assessed –
1
...
Single blind trials
3
...
Triple and quadruple trials
Advantages

1
...

3
...


It is reliable and valid
...

It ensures causality
...


Disadvantages

1
...

3
...

5
...


Ethical problem
It is expensive
It often needs more time and large sample
...

Loss to follow up
...


29

Field trials: Field trials, in contrast to randomized controlled trials, involve people who are healthy but presumed to
be at risk
...

Field trials are logically complicated and expensive
...

Community trials: In this type of studies, the treatment groups are communities rather than individuals
...
Cardiovascular disease is a good example of a condition appropriate for community trials
...

2
...

It is difficult to isolate the communities where intervention is taking place from general social changes that
may be occurring
...

Criteria:
1
...

3
...


Screening test do not usually establish a diagnosis
...

Screening test itself is very unlikely to cause harm
...


Examples:
1
...

3
...

5
...

7
...

9
...


A skin test called the tuberculin or PPD test, for detecting tuberculosis
...

The Pap smear test for cervical dysplasia or cervical cancer
...

The mammogram for identifying the possible presence of breast cancer
...

Fasting blood cholesterol for heart disease
...

Blood pressure for hypertension
...


WHO criteria for disease to be screened:

1
...

2
...

3
...

4
...

5
...

6
...

7
...

8
...

9
...

10
...

Wilson’s criteria for screening test:

1
...

2
...

30

3
...

4
...

6
...

8
...


specific
...

Treatment should be more effective if started early
...

Diagnosis and treatment should be cost-effective
...


Types of screening:

1
...
Examples:

2
...


• Visual defects in school children
...

• Newborn screening program in Japan
...
Examples:
• Sickle cell anemia
• Glucose tolerance test
Targeted screening: It is that type of screening where groups with specific exposures, occupational groups
are screened
...


4
...
Example: RHD in children
...

2
...

4
...
g
...

Control of disease: It is prospective screening; people are screened for the benefit of others e
...
HIV,
STI
...

Education: Screening raises public awareness
...

2
...

4
...


Tests are simple and safe
...

Facilities are easily available or easily provided
...

Equity: Equity to access to screening services; effective, acceptable and safe treatment are available
...

Negative predictive value: It is the probability of the person not having the disease when the test is negative
...

Specificity: Specificity is the ability of a test to exclude those who do not have disease
...


Disease + ve

Disease - ve

Total
Disease

Test + ve

Test - ve

Total Test

240

25

a

b

15

220

c

d

255

245

Answer:
Sensitivity

= a/(a+c) = 94
...
8%

Positive predictive value = a/(a+b) = 90
...
6%

32

265

235

500

33

6
...

Purpose of surveillance:

1
...
e
...

3
...

To provide scientific baseline data and information for priority setting, planning, implementing and evaluating
disease control program for both communicable and non-communicable health problems
...


Types:

A
...
Passive
They are discussed below:
A
...
It involves collection of data from communities such as in house to house
surveys or mobilizing communities to some central point where data can be collected
...

Example: Investigation of outbreaks
...
The collected data is complete and accurate
...
Information is timely collected
...

2
...

4
...


It requires good organization
...

It requires skilled human power
...

It is directed towards specific disease conditions
...
Unlike passive surveillance, it cannot be used for routine purposes
...
These conditions are –

1
...

2
...

3
...

New mode of transmission
...


When a previously eradicated disease reappears
...
Passive surveillance: Passive surveillance may be defined as a mechanism for routine surveillance based on
passive case detection and on the routine recording and reporting system
...
It involves
collection of data as part of routine provision of health services
...

2
...

4
...

Does not require special arrangements
...

Covers a wider area
...

2
...

4
...

Most of the time, data from passive surveillance is not available on time
...

It lacks representativeness of the whole population since passive surveillance is mainly based on health
institution reports
...

2
...

4
...

Data compilation, analysis and interpretation
...

Dissemination of information
...

2
...

4
...

Timely notification
...

Availability of a strong laboratory service for accurate diagnosis of cases
...
In this strategy, several activities from the
different vertical programs are coordinated and streamlined in order to make best use of scarce resources
...

Integrated disease surveillance strategy recommends coordination and integration of surveillance activities for diseases
of public health importance
...
Measures of Association
Probability: It is the proportion (%) of times an event would occur if an observation was repeated many times
...

Odds: It is the probability of an event divided by the probability of the event not happening
...
61
Odds ratio: An odds ratio (OR) is a measure of association between an exposure and an outcome
...
Odds ratios are most commonly used in case control studies
...

Develop disease

Don’t develop
disease

Totals

Incidence of disease

Smoke cigarettes

84

2916

3000

28
...
4 per 1000

37

PAR:
Proportion of Incidence Attributable to Exposure (Proportional Attributable Risk)
=
=

𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑖𝑛 𝑒𝑥𝑝𝑜𝑠𝑒𝑑 𝑔𝑟𝑜𝑢𝑝−𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑖𝑛 𝑛𝑜𝑛𝑒𝑥𝑝𝑜𝑠𝑒𝑑 𝑔𝑟𝑜𝑢𝑝
𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑖𝑛 𝑒𝑥𝑝𝑜𝑠𝑒𝑑 𝑔𝑟𝑜𝑢𝑝
𝐼𝐴𝑅
𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑖𝑛 𝑒𝑥𝑝𝑜𝑠𝑒𝑑 𝑔𝑟𝑜𝑢𝑝

Example:

IAR = 28
...
4
10
...
0−17
...
379
= 37
...

Odds: It is the ratio of the probability of having an event to the probability of not having the event
...


1−𝑝

Example: 1 out of 5 patients suffer a stroke
...


38

Interpretation of relative risk or risk ratio (RR):
•
•
•

RR = 1 suggests that, Risk in exposed is equal to risk in non-exposed
...


Interpretation of ODDs ratio
•
•
•

OR = 1 suggests that, odds of exposure is the same for cases and controls (no association between disease and
exposure)
...


Stating ODDs ratio results and Relative Risk (Risk Ratio) results:
OR = 5
...
44 times as likely to have had the exposure compared to those without the disease
...
41
Those with the exposure are 4
...


39

10
...
Some diseases are usually present at a
predictable level
...
But sometimes they occur in excess of what is unexpected
...
When the disease occur as epidemic, outbreak and pandemic, it is considered as excess of what is expected
...


1
...
When the exposure is
simultaneous, the resulting cases develop within one incubation period of the disease and this is called a point
source epidemic
...

Food borne epidemic following an event where the food was served to many people is a good example of point
source epidemic
...
A waterborne outbreak that spreads through a contaminated community water supply is an
example of a common source epidemic with continuous exposure
...


2
...
It can
occur through direct person to person transmission or it can involve more complex cycles in which the agent
must pass through a vector as in malaria
...
An outbreak of malaria is a good example of propagated
epidemic
...


3
...
Many food-borne pathogens result in mixed epidemics
...
The following questions should be
answered when investigating an epidemic:
•
•
•
•
•

What is the etiological agent responsible for the epidemic?
What is/are the predominant modes of transmission?
What specific source(s) of disease can be identified? Examples: Human carriers, breeding sites for vectors etc
...

What is the chain of events that led to the outbreak? Example: Accumulation of susceptible hosts in an area
...
Through timely analysis of routine surveillance data
...
Report from clinician
...
Report from the community, either from the affected group or concerned citizen
...


1
...

Preparation can include –
• Investigation must have the appropriate scientific knowledge, supplies and equipment to carry out the
investigation
...

Hence, he/she should inform and involve other high level health professionals from the outset
...

• Arrange transportation and organize personnel matters
...
Verify (confirm) the existence of an epidemic: This initial determination is often made on the basis of
available data
...
Instead of comparing absolute numbers, it is advisable to
compare rates like incidence rate
...
Verify (confirm the diagnosis): One has to always consider whether the initial reports are correct
...
For example, the adready collected blood
film slides can be seen by laboratory experts to check whether the initial report was correct
...

The importance of the index case and other early cases for diseases that are known to occur in epidemic form,
such as relapsing fever, is an indication to health authorities of the possible start of an outbreak
...


4
...
Case
definition is defined as a standard set of criteria to differentiate between cases and non-cases
...
Describe the epidemic with respect to person, place and time: Each case must be defined according to
standard epidemic parameters: the date of onset of the illness, the place where the person lives or become ill
and the sociodemographic characteristics (age, sex, education level, occupation)
...
Identify the causes of the epidemic: All factors that can contribute to the occurrence of the epidemic should
be assessed
...
Management of epidemic and follow up: Although it is discussed late, investigation must start as soon as
possible depending on the specific circumstances
...
For example, an outbreak might be controlled by destroying contaminated foods, disinfecting
contaminated water, destroying mosquito breeding sites or an infectious food handler could be suspended from
the job and treated
...
Write down the differences between case control study and cohort study
...

Starts with the disease
...

2
...

4
...

6
...

8
...

10
...

12
...


1
...


Tests whether the suspected cause occurs more
frequently in those with the disease than among
those without the disease
...

Involves fewer number of subjects
...

Suitable for the study of rare diseases
...


3
...

Temporal association is never proven
...

Relatively inexpensive
...


9
...

Starts with people exposure to risk factors or
suspected cause
...


4
...


5
...

7
...


Involves larger number of subjects
...

Not suitable for the study of rare diseases
...

Can yield information about more than one
disease outcome
...

Recall bias is not an issue
...

Example: Smoking causes lung cancer
...

11
...

13
...
What are the differences between observational study and experimental study?
Differences between observational study and experimental study:
1
...

3
...

5
...

7
...


Observational study
In observational study, the researcher observes
and systematically collects information
...

In this type of study, the researcher relies only
on the data collection
...

Here, random data is used
...

It may be descriptive and analytical
...


1
...

3
...

5
...

7
...


Experimental study
In experimental study, the researcher intervenes
to change something
...

In this type of study, the researcher relies both
on experiment and data collection
...

Here, random data is not used
...

It may be true experimental and quasi
experimental
...


Q
...

Differences between incidence and prevalence:
1
...


42

Prevalence
Prevalence is the actual number of cases alive; with
the disease, either during a period of time (period

2
...

4
...

6
...


What
is
measured
Numerator
Denominator
Type of study
Time
of
disease
diagnosis
Uses

Rapidity of disease occurrence
...

Proportion of population with disease
...

Population at risk
...

Whole population
...


ii
...

Useful in the study of the
burden of chronic diseases
and implication for health
services
...

ii
...


Expresses the risk of becoming ill
...

More useful for studies of causation
...
What is descriptive epidemiology? How it differs from analytical epidemiology?
Descriptive epidemiology: Descriptive epidemiology is one of the basic types of epidemiology which is concerned
with describing the frequency and distribution of diseases and other health related conditions by time, place and person
...


2
...

4
...

6
...

Generates hypothesis
...

Can be a case report, case series and cross
sectional studies
...

Tools include
dimensions
...


2
...


Tests hypothesis
...


4
...

Enables risk assessment of disease among
populations
...


5
...


Ensures understanding of the basic dimensions
of a disease
...
It is cheap and quick
...
There is no information on the individual
...
No control for confounding
...
Results are more ambiguous
...


7
...


Ensures incidence rates and prevalence of
diseases in populations
...
It is expensive
...
There is information on the individual
...
Control for confounding
...
Results are less ambiguous
...
Write down the differences between screening and diagnostic test
...


Screening test
Screening is the process of using tests on a large
scale to identify the presence of disease in
apparently healthy people
...


43

Diagnostic test
Diagnostic test is the use of clinical and/or
laboratory procedure to confirm or refute
the existence of disease or true abnormality
in patients with signs and symptoms
presumed to be caused by the disease
...

3
...

5
...

Applied to groups
...

Based on one criteria and cut-off
...

7
...

9
...


It is less accurate
...

It is not used as a basis for treatment
...

Example: Fasting blood sugar for diabetes,
fasting blood cholesterol for heart disease, blood
pressure for hypertension etc
...

3
...

5
...

Applied to single patient
...

Based on evaluation of a number of signs
and symptoms and laboratory findings
...
It is more accurate
...
It is more expensive
...
It is used as a basis for treatment
...
Its initiative comes from a patient
...
Example: Complete blood count (CBC),
Widal test, MRI, Western blot test etc
...
An epidemic refers to a
contagious, infectious or viral illness that spreads to many people in one specific geographic region
...

Types:
1
...
Propagated or progressive epidemic and
3
...


2
...


Common source epidemic: Disease occurs as a result of exposure of a group of susceptible persons to a
common source of a pathogen, often at the same time or within a brief time period
...
Food borne epidemic following an event where the food was served to many people is a good
example of point source epidemic
...
A waterborne outbreak that spreads through a contaminated
community water supply is an example of a common source epidemic with continuous exposure
...
It can occur
through direct person to person transmission or it can involve more complex cycles in which the agent must
pass through a vector as in malaria
...

Mixed epidemic: The epidemic begins with a single, common source of an infectious agent with subsequent
propagated spread
...


Causes: There are several changes that may occur in an infectious agent that may trigger an epidemic
...

Certain epidemics occur at certain seasons
...


Pandemic: The term pandemic comes from the Latin words pan (all) and demos (people)
...
A pandemic is not limited to
one specific geographic region
...

Phases of a Pandemic:
•
•

•

Phase 1: During phase 1, viruses circulate within animals only
...

Phase 2: In phase 2, an animal virus has caused an infection in a human being
...
Humans are
now susceptible to the virus
...
Groups of human beings have contracted the virus in one
community
...
At this point, the illness may be considered to be an epidemic in that
community, but it is not yet pandemic
...
Human to human and animal to human virus transmission
are causing outbreaks in many communities and more people are getting sick in those communities
...

Phase 5: In phase 5, human to human transmission is taking place in at least two countries in one WHO
region
...
At phase 5, a majority
of countries have not yet been affected, but a pandemic is considered imminent
...

Phase 6: The last stage is stage 6
...
Illness is widespread and
governments and health officials are actively working to curtail the spread of the disease and to help their
populations deal with it using preventive and stop-gap measures
...
After the increase in activity,
the disease spreading activity will begin to wane
...


The time frame of these phases varies greatly, as it could range from months to years
...

2
...

4
...

6
...

Increased absenteeism in all sectors of the workforce
...

House quarantine
...

Reduction in non-essential services
...

•
•
•
•

A disease is an interruption, cessation or disorder of body functions, systems or organs (e
...
, cholera, angina,
breast cancer, influenza)
...
g
...

A behavior is a manner of conducting oneself (e
...
, physical activity, diet, safety precautions)
...
g
...


Health related states or events play a vital role in epidemiology because these all play a role in the condition of a person
to be ill or not
...


Agent factors
Agent: The agent has been defined as an element, a substance or a force either animate or inanimate, the presence or
absence of which may, following effective contact with the susceptible human host and under proper environmental
conditions, serve as a stimulus to initiate or perpetuate a disease process
...


Biological agents: These are living agents of disease
...

9
...
Any excess or
deficiency of the intake of nutritive elements may result in nutritional disorders
...
Chemical agents: These may be endogenous or exogenous:
• Endogenous: Chemicals produced inside the human body as a result of derangement of function are
called endogenous chemical agents
...


46

•

11
...

13
...


Exogenous: Agents arising outside the human host are called exogenous chemical agents e
...
,
allergens, fumes, dust etc
...

Social agents: These include poverty, smoking, abuse of drugs and alcohol, unhealthy life styles, social
isolations, maternal deprivation
...

Psychological agents: Work stress is an example of psychological agent
...
Reservoirs for infectious agents may be humans, animals, plants or other inanimate objects
...

Types: Reservoirs can be classified into the following three main types:
4
...

6
...

5
...


Human reservoirs: Human reservoirs are human beings infected by pathogens that exist on or within the
human body
...

Animal (non-human) reservoirs: Animal or non-human reservoirs consist of domesticated and wild animals
infected by pathogens
...
Virus of the taxon Ebolavirus, which causes Ebola
virus disease are thought to have a natural reservoir in bats or other animals exposed to the virus
...
These reservoirs may exist on land (plants and soil), in
water or in the air
...
The bacteria Legionella
pneumophila, a facultative intracellular parasite which causes Legionnaries disease and Virbio choleare, which
causes cholera, can both exist as free-living parasites in certain water sources as well as in invertebrate animal
hosts
...
For example,
a disease like smallpox could be eradicated from this planet, in part because humans were the main reservoir
...
This is one of public health’s great
achievements and currently similar attempts are underway to do the same with poliovirus

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