DEFINITION, SCOPE, and CLASSIFICATION
Epidemiology is the study of the distribution
and determinants of both disease and injury. Two triads are involved in epidemiology: (a) the agent, host, and environment
triad and the time, place, and person triad. The primary goals of epidemiology are prevention, control, and, in rare instances,
eradication disease and injury. Epidemiology started as a study of epidemics and extended to cover infectious disease and
later non-infectious diseases. It has now become a methodological discipline that is used to study disease and non-disease
Qualitative epidemiology deals with
qualitative descriptions. Quantitative epidemiology deals with numerical descriptions. Observational epidemiology is based
on observation of human phenomena. Experimental epidemiology involves assessment of the effects of intervention against a
disease phenomenon. Theoretical epidemiology deals with mathematical and methodological issues. Descriptive epidemiology describes
the patterns of disease occurrence in terms of place, time and person. Analytic epidemiology seeks to discover the underlying
causes of diseases.
Public-health epidemiology deals with
preventive medicine. Clinical epidemiology deals with diagnosis, management, and prognosis of disease. Hospital epidemiology
deals with nosocomial infections and other aspects of hospital operations that can be studied using epidemiological methodology.
Drug or pharmaco-epidemiology studies phenomena of adverse reactions and side-effects of drugs. Genetic epidemiology studies
the patterns of inheritance of disease from parents and how genetic and environmental factors interact in the final pathway
of disease causation. Molecular epidemiology deals with phenomena at the molecular level. Occupational epidemiology studies
diseases due to exposure to hazardous material or working conditions in the work-place. Environmental epidemiology studies
the impact of air, water, and soil pollution on health. The supporting disciplines of epidemiology are clinical sciences,
demographical sciences, data and information sciences, behavioral sciences, and environmental sciences.
IMPORTANCE and PIONEERS OF EPIDEMIOLOGY
Epidemiology is used in clinical medicine,
public health, and actuarial sciences. The major activities of an epidemiologist are: study design including selection of
the study sample, data collection, data analysis, data interpretation, and initiation of action programs to prevent disease
and promote health. Professional practice and careers in Epidemiology are in government (Ministry of Health), universities,
hospitals, and the private sector (drug manufacturers), and research institutes. Famous epidemiologists contributed to the
early growth of the discipline. Hippocrates made the first recorded epidemiological observations by describing the relation
of disease to climate and geography. John Snow (1813-1858) recognized the importance of field epidemiology in his study of
cholera and its relation to water pollution William Budd (1811-1880) described the spread of typhoid due to ingestion of infected
material from patients. William Furr realized that cycles of epidemics could be described mathematically. Major Greenwood
(1880-1949) was chief of epidemiology and vital statistics at the London School of Hygiene and Tropical Medicine worked on
models of epidemics.
HISTORICAL EVOLUTION OF EPIDEMIOLOGIC KNOWLEDGE
Five stages can be identified in the evolution of epidemiological knowledge. The ancient period up
to 1500, the post renaissance period 1500-1750, the sanitary period 1750-1870, the infectious disease period 1870-1945, and
modern epidemiology period starting in 1945 (also considered the chronic disease period).
In the ancient period, inter-personal disease transmission, connection between diseases and the environment,
quarantine and isolation were known. In 400 BC Hippocrates suggested the relation between disease on one side and lifestyle
and environmental factors on the other side.
The post renaissance period witnessed rapid growth of knowledge of pathology, and transmission as well
as control of disease. In the 1660s Bacon and others developed inductive logic that provided a philosophical basis for epidemiology.
Girolamo Fracastoro (1478-1553) suggested that disease spread by direct contact and by small living particles. In 1683 Van
Leeuwenhoek saw microorganisms under the microscope. In 1662 Captain John Graunt analyzed births and deaths and described
disease in population quantitatively with significant epidemiological observations and determinations. In 1747 James Lind
discovered the prevention of scurvy by conducting one of the first experimental trials on humans. In 1798 Edward Jenner discovered
vaccination. Ramazzini wrote on occupational health in 1770. Percival Pott (1713-1788) associated scrotal cancer to chimney
In the sanitary period concern was about environmental correlates of disease; quarantine and isolation
were used for disease control.
During the infectious disease period, the microbial basis of disease became firmly established when
Louis Pasteur (1822-1895) and Robert Koch (1843-1900) developed the germ theory through experimentation. Dr Robert Koch the
father of bacteriology identified causative organisms of anthrax (1876), tuberculosis (1882), and cholera (1883). He developed
Koch’s postulates which were criteria for determining an infectious etiology of disease. In 1847 Ignaz Philip Semmelweis
suggested hand-washing to avoid obstetric infection. John Snow described the association between cholera and contaminated
water by forming and testing a series of hypotheses thus being a pioneer of analytic epidemiology. William Budd in 1857-73
concluded that typhoid was contagious. In 1839 William Farr started the discipline of vital statistics as a system of regular
collection and interpretation of data and set up a system for routine summaries of causes of death. Joseph Lister introduced
antiseptic surgery in 1865. Manson Barr, Bruce-Chwatt and others studied the transmission of mosquito-borne infections, malaria
and yellow fever.
Towards the end of the infectious disease period, there were developments in knowledge of non-infectious
disease and statistical methodology. Non-infectious diseases (nutritional, occupational, psychiatric, and environmental) were
identified and were studied. In 1905 beriberi was found associated with eating milled rice. In 1920 Joseph Goldberger published
a descriptive field study relating pellagra to diets high in cereal & canned foods and free of fresh animal products.
Elmer McCollum a Professor at Johns Hopkins since 1918 discovered vitamin-deficiency diseases. Statistical theory and practice
developed rapidly towards the close of the 19th century to keep up with developments in basic research and public
health all of which required statistical analysis.
The period of modern epidemiology starting in 1945 is the chronic disease epoch. By 1945 there was
convergence of the non-mechanistic concepts of disease (environment, social, and behavioral basis of disease) and the mechanistic
concepts of disease (molecular, biological, gent-host interaction). Health was defined in a broad sense as: physical, mental,
psychological, and spiritual well-being. Scientists recognized the multi-causal nature of disease (genetic, psycho-social,
physiological, and metabolic). The period is witnessed a demographic transition (ageing populations) as an epidemiologic transition
(change from communicable to non-communicabe diseases). It also witnessed major studies that helped redefine the direction
of epidemiology and public health. In 1949 the Framingham Heart Study was began as the first cohort study of the causative
factors of cardiovascular disease. In 1950 Doll and Hill, Levin et al, Schreck et al. and Wynder and Graham published the
first case control studies of smoking and lung cancer. In 1954 the Field trials of the Salk polio vaccine were the largest
formal human experiment. In 1971-1972 the North Karelia Project and the Stanford Three Community studies were launched as
the first community-based cardiovascular disease prevention programs. Further methodological developments were witnessed in
this period. In 1960 MacMahon published the first epidemiology textbook with systematic treatment of study design. In 1959
Mantel and Haenszel developed statistical procedures for case control studies. In the 1970s logistic regression and log-linear
regression were developed as new multivariate analytic methods. In the 1970s – present new developments in computer
hardware and software. In the 1990s molecular techniques are being applied to study of large populations.
ETHICO-LEGAL ISSUES IN EPIDEMIOLOGY
A study involving humans must get approval
from a recognized body. For approval the study must fulfil certain criteria. It must be scientifically valid. It is unethical
to waste resources (time and money) on a study that will give invalid conclusions. In 1992 the Council for International Organizations
of the Medical Sciences published ‘Guidelines for Ethical Review of Epidemiological Studies. Among ethical considerations
are: individual vs. community rights, benefits vs. risks, informed consent, privacy and confidentiality, and conflict of interest.
Study interpretation and communication
of findings to the public pose problems. Risk reports that are not yet confirmed are picked up by the media and create unnecessary
public concern. Study findings affect policy. Epidemiologists must know how to communicate risk to the public. It is an ethical
obligation to report research findings to subjects so that they may take measures to lessen risk. Epidemiological evidence
is different from legal evidence. Epidemiological evidence may not be accepted in a court of law because it has few certainties;
it is concerned with populations whereas legal