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The prevalence of a disease is the number of diseased individuals at any one time (point prevalence) or over a given period (period prevalence). The incidence is the number of new cases of a disease that occur within a defined population over an established period of time. Frequently either prevalence or incidence, or both, are given as a rate, meaning the number of cases in a fixed number of people, e.g., cases per 100,000. Individual cases of disease in widely separated geographic areas or otherwise independent cases are said to be sporadic. Any excessive and related incidence of a particular disease above what is normally expected in a population is defined to be an epidemic.

The term epidemic has been derived from two Greek words, "Epi" means "upon " and "Demos" means "people". Thus, an epidemic of an infectious disease is the occurrence of a number of cases of a disease, known or suspected to be of infectious origin, that is unusually large or unexpected for the given place or time. An epidemic often evolves rapidly. A threatened (potential) epidemic is said to exist when the circumstances are such that epidemic occurrence of a specific disease may reasonably be anticipated. This requires a susceptible human population, the presence of a disease agent, and the presence of a mechanism or mode of large-scale transmission (e.g., contaminated water supply, poor sanitation and vector population) The unusual occurrence in a community or region of disease, specific health related behaviours. Some use the term "outbreak" for a small, usually localized epidemic in the interest of minimizing public alarm, unless the number of cases is indeed very large. This definition covers the usual epidemic diseases such as, measles, chickenpox, and cholera, which are compressed in time, but also the modern "slow" epidemics of non-communicable diseases like diabetic, heart attacks, and depression. Epidemics are Public Health emergencies.

When an epidemic extends beyond the confines of a wide area, typically a continent, and becomes a more widespread problem, it is a pandemic.

AIDS and Avian Flu are pandemic diseases today. Any disease with a low to moderate normal base level incidence rate in the population, but not necessarily constant, is said to be endemic. The common cold is endemic in northern latitudes. Cholera and malaria are endemic diseases in some parts of India. Infectious diseases are major causes of morbidity and mortality in many parts of the region.

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