Epidemiology: An Overview
Dr. Bhoj R Singh, Principal Scientist (VM)
I/C Epidemiology; Centre for Animal Disease Research and Diagnosis
Indian Veterinary Research Institute, Izatnagar-243122, Bareilly, UP,
Father of Epidemiology: Hippocrates, 400 BC
The first person known to have examined the
relationships between the occurrence of disease and
He coined the term ‘epidemic’ in his essay entitled “On
Airs, Waters, and Places,”
Clinicians are concerned with the health of an individual;
epidemiologists are concerned with the collective health of
the population in one or other area.
• Hippcrates (400 BC) an essay entitled “On Airs, Waters, and Places,”.
• Another early contributor to epidemiology was John Graunt, a
London haberdasher who published his landmark analysis of
mortality data in 1662. He was the first to quantify patterns of birth,
death, and disease occurrence, noting male-female disparities, high
infant mortality, urbanrural differences, and seasonal variations.
• In 1854, William Farr wrote a book ‘Cholera’. He began to
systematically collect and analyze Britain’s mortality statistics. Farr,
considered the father of modern vital statistics and surveillance,
developed many of the basic practices used today in vital statistics
and disease classification. He extended the epidemiologic analysis
of morbidity and mortality data, looking at the effects of marital
status, occupation, and altitude. He also developed many
epidemiologic concepts and techniques still in use today.
• Epidemiology= (Epi=upon/ among+demos=people/
• Epizootiology= epi=upon+zoo=animal+logos=discourse)
• Epidemic= outbreak of disease in human population
• Epizootic= outbreak of disease in animal population
• Epornitics= outbreak of disease in bird population
• Epidemein= to visit a community, I.e., a diseases which comes periodically,
not present continuously
• Endemein= resides within (the diseases continuously present in a
• Epidemiology is the study of health-event, health-characteristic, or health-
determinant patterns in a population.
• “Epidemiology is the study of the distribution and determinants of health-
related states or events in specified populations, and the application of
this study to the control of health problems.” (Last JM, 1988)
• Mayer inferentially acknowledges the epidemiologic character of
population phenomena, for as its etymology indicates, (epi, upon;
demos, people; logos, study), epidemiology is the study of what
“comes upon” groups of people. More specifically epidemiology is
concerned with the distribution of disease and death, and with
their determinants and consequences in population groups.
• Epidemiology is: a) a quantitative basic science built on a working
knowledge of probability, statistics, and sound research methods; b)
a method of causal reasoning based on developing and testing
hypotheses pertaining to occurrence and prevention of morbidity
and mortality; and c) a tool for public health action to promote and
protect the public’s health based on science, causal reasoning, and a
dose of practical common sense. (Cates WJ. 1982)
• the study of the distribution and determinates of health related
states in specified populations and the application of this study to
control health problems (Peterson DR 1970)
Veterinary Epidemiology, Epizootology
• Investigation and assessment of health and
productivity related events in animal
Objectives of Epidemiology
1. To determine origin of disease in a population
when etiology is known
2. To investigate and control a disease where
etiology is obscure or little understood
3. To determine ecology and natural history of a
4. Planning and monitoring of disease control
5. Assessment of economic effects of a disease and
cost/benefit of a control programme
Uses of Epidemiology
• to identify the etiology or cause of a disease and the risk factors."
• "to study the natural history and prognosis of disease".
• "to evaluate new models of health care delivery."
• "to provide a scientific foundation for regulatory decisions relating
to health or environmental problems.
• "to provide a clue to changes taking place over time…"
• "to identify subgroups in the population who are at high risk for
• to determine the best or most appropriate types of primary and
– primary prevention prevents disease in healthy individual
– Secondary prevention limits disease by early detention usually
through screening programs.
Types of Epidemiological
Disciplines of Epidemiology
• Computational epidemiology
• Genetic epidemiology
• Molecular epidemiology
• Environmental epidemiology
• Nutritional epidemiology
• Applied Epidemiology
– Clinical epidemiology
– Micro-epidemiology (Study of disease in small group of
individuals as study of feline AIDS in kittens and then application
to understand human AIDS)= Comparative epidemiology
– Macro epidemiology (Study of disease in population in national
and international perspective)
• The science of characterization of the distribution
of health-related states or events is one broad
aspect of epidemiology called descriptive
epidemiology. Descriptive epidemiology provides
the What, Who, When, and Where of health-
• We temporarily classify a case as suspect or
probable until laboratory results are available.
• When we receive the laboratory report, we then
reclassify the case as either confirmed or “not a
case,” depending on the lab results.
• Analytic epidemiology attempts to provide the
Why and How of such events by comparing
groups with different rates of disease occurrence
and with differences in demographic
characteristics, genetic or immunologic make-up,
behaviours, environmental exposures, and other
so-called potential risk factors. Under ideal
circumstances, epidemiologic findings provide
sufficient evidence to direct swift and effective
public health control and prevention measures.
• The term applied epidemiology is sometimes used to
describe the application or practice of epidemiology to
address public health issues. Examples of applied
epidemiology include the following:
• the monitoring of reports of communicable diseases in
• the study of whether a particular dietary component
influences risk of developing a disease
• evaluation of the effectiveness and impact of a health
awareness or production improvement program
• analysis of historical trends and current data to project
future health and production resource needs
Concepts of Disease causation
• Demons: (Placation- sacrifice, exorcism, evasion-millet seeds,
transference- scapegoats, Gadarene swine, amulets, talismans,
fetishes and icons.
• Divine wrath: From Old testament, Persian and Aztec writings.
• Metaphysical: Stars, planets (treated with foul medicine, swallowing
of toads, fishes, moles roasted alive).
– Derangement of four humours of body (Geeks; Heat, moisture,
dryness and cold; air, earth, water and fire) creating mismata
– Vedic theory, three humors and tridosha (Vat, pitta, cough)
• Contagion (Contagium animatum- Edward Jenner, 17th Century;
contagium vivum fluidum- virus, Beijerinck, 1998-99)
Studies in Epidemiology
1. Experimental (Full control of investigator on formation
of groups, clinical trials, nutritional trials)
2. Cross-sectional: Investigator tries to find a relation
between occurrence of disease with a hypothesized
3. Case-Control: Investigator compares groups of diseased
with healthy population with respect to exposure to a
hypothesized causal agent.
4. Co-hort: Investigator compares two groups one
exposed to hypothesized cause and other not exposed
for occurrence of disease (Prospective)
Methods in epidemiology
• Survey: Examination of aggregates of units of disease in population i.e., record of
• Types of Surveys
– Sample Survey (On a sample of population)
– Census (On total population)
– Cross sectional (record of events a defined point of time)
– Longitudinal (record of events over a period of time): Prospective (present to future),
Retrospective (records of past events)
– Screening: A kind of survey for identification of undiagnosed cases of a disease often with
some rapid test/ examination.
• Monitoring: Routine observation on health/ production of a population and
environmental factors, without much emphasis on identification of a diseased
animals. (Regular meat/ milk recording, abattoir examination, meat inspection
• Surveillance: More intensive form of monitoring often associated with a control
programme, identification of an individual with disease in view to detect changes
in population and its environmental health.
• Sentinel units: Units selected for monitoring and surveillance as abattoirs, some
clinics, farms etc), horses may be sentinel for VEE, stray dogs for Canine distemper)
• Study in epidemiology means comparison of groups for occurance of disease/
production, thus survey is not a study.