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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. 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.


Contamination
Contamination:
The presence of an infectious agent on a body surface, also on or in clothes, beddings, toys, surgical instruments or dressings, or other inanimate articles or substances including water, milk and food.
Infestation:
For persons or animals the lodgment, development and reproduction of arthropods on the surface of the body or in the clothing, e.g. lice, itch mite.
Infectious Diseases:
A clinically manifest disease of man or animals resulting from an infection.
Contagious Disease:
A disease that is transmitted through contact. Examples include scabies, trachoma, STD and leprosy.

Communicable Diseases:
An illness due to a specific infectious agent or its toxic products capable of being directly or indirectly transmitted from man to man, animal to animal or from the environment through air, dust, soil, water, food etc.
Epidemic:
The unusual occurrence in a community or region of disease, specific health–related behavior clearly in excess of “Expected occurrence”. It applies to infectious diseases as well as the “Slow modern epidemics” such as heart disease cancer etc.
Endemic:
It refers the constant presence of a disease or infectious agent within a given geographic area or population group, without importation from outside. For e.g. in some areas malaria has a constant presence called endemic malaria.
Zoonosis:
An infection or infectious disease transmissible under natural conditions from vertebrate animals to man e.g. rabies, plague, bovine tuberculosis.
Nosocomial infection:
Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or other health care facility. It includes infections acquired in the hospital but appearing after discharge, and also such infections which affects the working staff.
Opportunistic infection:
This is infection by a organism(s) that takes the opportunity provided by a defect in host defense to infect the host and hence cause disease. It is commonly seen in people whose immunity is compromised such as people who are on steroids.

Latrogenic Disease:
Any untoward or adverse consequence of a preventive, diagnostic or therapeutic regimen or procedure, that causes impairment, handicap, disability or death resulting from a physician’s professional activity or from the professional activity of other health professionals.