Epidemiology is the basic science of medicine. It is fundamental to practice of clinical medicine and public health. Study of health related states and diseases in individuals, its effects on person and community, its progression in nature, its cause; all are important aspects in the study of medicine.
Basic understanding of concepts of epidemiology, process of epidemiological reasoning, methods and application, will facilitate understanding health and disease.
Historical Overview of Epidemiology
The history of epidemiology is as old as that of medicine. The writing of Hippocrates (fourth and fifth century BC) has mention of careful observations on “Air, Waters and Places”. In Aurveda, spread of diseases through food and water, transmission of disease from person-to-person, were carefully studied and preventive measures are also mentioned.
In modern epidemiology, Sir John Snow was pioneer in application of epidemiological investigations. He was one of the founder members of London Epidemiological Society. He investigated the occurrence of cholera in London between 1849-1854. In 1854, Snow noted that cholera cases were particularly high in areas, which received water supply from South Walk and Vauxhall Company. This company used to draw water from a point in Thames River and it was heavily 2polluted with sewage. Another company – Lambeth Company also supplied water to London residents. John Snow investigated epidemic and calculated case rate and mortality rate from the houses supplied with water by these two companies and compared with rates of London. He found that mortality from cholera was eight-to-nine times higher in houses supplied by South Walk and Vauxhall Company. It lead to legislation in 1857, that all water companies mandatory filter water before supplying to citizens. Robert Koch identified cholera organism many years later in 1883.
The term epidemiology was originally used in relation to study of epidemic of infectious diseases. Until late nineteenth century infectious diseases like tuberculosis, smallpox, typhoid, plague, cholera affected major parts of the world. Later on many of them were controlled in developed countries and there was change in mortality patterns. The changes in disease pattern occur worldwide. The chronic diseases emerged as major health problem.
Since World War II, systematic progress in science of epidemiology occurred. The epidemiological studies were no longer restricted to infectious diseases. Subsequently epidemiological methods were extensively used to study diseases with long latency period like heart diseases, diabetes mellitus, and cancers. Epidemiological studies of risk factors have largely contributed in understanding non-communicable diseases. The present day epidemiology has become integral part of clinical research, planning and development of health services and practice of medicine.
DEFINITION, SCOPE AND USES OF EPIDEMIOLOGY
Definition
Epidemiology is defined as “study of the distribution and determinants of health related states and events in specified population and the application of this study to control of health problem.” This is most widely accepted definition, given by John Last (1988).
3Health related states could be diseases, disorders, defects, impairments. It could be physiological conditions like pregnancy, child birth or menopause. It could be habits or substance use or behavior which can affect health.
Health related events could be birth, death, accidents or injuries.
In above definition, the term distribution relates to occurrence of particular health related condition in human populations. This occurrence is studies in relation to time, place and persons.
The second part is determinants of particular health related state or disease, which means searching for factors which directly or indirectly contribute towards occurrence of disease or health related condition.
The third part is application, since epidemiology is an applied science, the study results are has to be applicable for control of health problems.
Scope of Epidemiology
The modern epidemiology has evolved from early periods in history where natural experiments and observations were recorded. The inferences drawn from these observations were used for control of diseases. The scope of epidemiology should not be equated as uses of epidemiological methods. The knowledge and understanding of epidemiology is equally helpful to clinicians and researchers. In clinical practice epidemiological principles can be applied in clinical diagnosis and decision-making. It is also helpful in interpreting and drawing inferences from available clinical studies. Epidemiological principles and methods have been applied in various disciplines of medicine, viz. pharmacology, oncology, infectious and chronic diseases, genetics and immunology, etc. The public health specialists and policy makers also utilize the information generated by epidemiological studies for making health programs.4
Uses of Epidemiology
Historical Study
Study of changes in trends in disease pattern and mortality is first use of epidemiology. This has lead to many important decisions in relation to control of communicable diseases. It helps in understanding reasons for rise and fall in cases. We are aware that many diseases like cholera, plague, malaria which killed large number of persons in past are no longer so. The diseases those were silent for long emerged as a threat in many parts of the world. Yet new diseases have emerged. Study of historical changes helps to make future projections about emerging and re-emerging diseases.
Community Assessment
Identification of types of morbidity in the community and quantification of it is necessary for assessing needs of community in relation to health services. For instance, proportion of individuals suffering from tuberculosis, diabetes mellitus or hypertension can help in identifying need of community with respect to health services.
Assessment of community also helps to understand profile of illnesses and its relation to socioeconomic, cultural and environmental factors. A large number of children with protein energy malnutrition could be due to sociocultural, environmental and medical reasons. Community assessment also helps to predict future health problems, for instance large number of unimmunized children points toward possible danger of outbreak of disease in future. Children consuming excess calories and avoiding physical activities could be a pointer toward future problems of obesity and associated diseases.
Working of Health Services
Epidemiological studies help to evaluate the working of health services. It identifies the health problems in relation to time, place and persons and this is the basis of planning of health 5services. The efficient and effective health services reduce health problems in the community. Epidemiological studies are useful in assessing the changes in mortality and morbidity which are expected after implementation of a particular health program.
Assessment of Individual Risks and Chances
Individual risks of contracting disease are based on probabilities derived from previous studies. For an individual seeking life insurance, risk of death and disabilities are calculated. Five years survival rate of cancers, chances of survival after a particular surgery, cure rate of treatment regimen, are examples of assessment of risk and chances.
Identification of Syndromes
Epidemiology can be used to define syndromes. In recent years identification of AIDS was based on observation of cases of Kaposi's sarcoma in group of persons.
Completing the Natural History of Disease
Epidemiological observations can help in collecting different types of presentation of a disease. Based on it models can be constructed. Further studies help in completing the natural history.
Search for Causes
The most practical use of epidemiology is testing the hypothesis for identifying diseases or risk factors.
The contribution of epidemiology in identifying cause of disease is great.
To quote some, epidemiological studies showed causal association between smoking and lung cancer, radiation exposure and leukemia, maternal infection with rubella and congenital rubella syndrome, dental caries and fluoride in drinking water.6
Other Uses
Above mentioned uses were defined by Morris. Advancement in epidemiology has added further uses. These are related to clinical epidemiology
- Study of clinical signs and symptoms and development of clinical algorithm.
- Clinical decision-making based on clinical trials. Clinicians extensively use it for choosing effective regimen.