Epidemiology is a very ancient science. Epidemiology is derived from the word epidemic (epi=among, demos=people, logos=study). Therefore, Epidemiology is the basic science of preventive and social medicine. As a scientific definition it involves the studies of disease frequency, studies of the distribution and studies of the determinants of disease.

Importance of Epidemiology
Most of the important measures adopted by society has its roots in the study of the epidemiology of the disease. For e.g. the risk factors for heart disease were studied in a landmark study the Framingham heart study. Thus we know today that obesity, smoking, high cholesterol are all important in the prevention of heart attacks. Many such examples can be given.

Thus broadly we can say that epidemiology has the following aims.
To describe the distribution and magnitude of health and disease problems in human populations To identify risk factors for diseases. To provide the data essential to the planning, implementation and evaluation of services for the prevention, control and treatment of disease. The ultimate goal of epidemiology is to promote the health and well being of society as a whole. In clinical medicine, the physician is concerned with disease in the individual patient, whereas the epidemiologist is concerned with disease patterns in the entire population.

Use of statistics in Epidemiolgy.

• A number of statistics are used in epidemiology. We shall outline some of the important ones. These statistics deal with a number of factors.
• Measurement of mortality, morbidity, disability.
• Measurement of medical needs, health care facilities, utilization of health services and other health related events.
• Measurement of the presence, absence or distribution of the environmental and other factors suspected of causing the disease.
• Measurement of the demographic variables. Examples includes infant mortality rate, maternal mortality rate etc.

Let us look at some of these statistics:
Incidence:
Incidence rate is defined as “The number of new cases occurring in a defined population during a specified period of time”.
Prevalence:
The term “Disease prevalence” refers specifically to all current cases (old and new) existing at a given point in time, or over a period of time in a given population.
Infection:
The entry and development or multiplication of an infectious agent in the body of man and animals.