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 phenomena.

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 the London 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.

EPIDEMIOLOGIC METHODOLOGY:

An epidemiologic investigation proceeds through identifying and describing a problem, using the scientific method to formulate and test hypotheses, and interpreting findings. Epidemiological information is sourced from existing data or studies (observational or experimental). Existing data is from census, medical facilities, government, and private sector, health surveys, and vital statistics. Experimental studies, natural or true experiments, involve deliberate human action or intervention whose outcome is then observed. They have the advantage of controlled conditions but have ethical problems of experimenting on humans. Observational studies allow nature to take its course and just record the occurrences of disease and describe the what, where, when, and why of a disease. There are of 3 types of observational studies: cross-sectional, case control, and cohort (follow-up) studies.  Their advantage is low cost and fewer ethical issues. They suffer from 3 disadvantages: disease aetiology is not studied directly because the investigator does not manipulate the exposures, unavailability of information, and confounding.

Epidemiological methodology, following the scientific method, is empirical, inductive, and refutative. Epidemiology relies on and respects only empirical findings. Empiricism refers to reliance on physical proof. Induction is building a theory on several individual observations. Refutation is basically refusal of a supposition until it is proved otherwise. Epidemiological investigation is not as deterministic as laboratory investigation but is cheap and easy.

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 soot.

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 19^th 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.