All epidemics have the following common characteristics: i) An unexpected number of cases of particular disease occur at a particular point of time affecting large segment of population. ii) Generally confined to a definite population or geographical area and hence geographic patterns provide us important sources of clues about the causes of diseases.
The host and environment are in constant interaction and that a disease is caused by disturbance of equilibrium between agent, host and environment. The disease assumes epidemiological proportions when the environmental conditions are favorable for the disease agent and unfavourable conditions exist for man.
Epidemic is a derivation of two Greek word epic (upon / among) and demos (people). It is the ‘unusual’ occurrence in a community or region of a disease specific health related events “clearly in excess” of the “expected occurrence”.