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The rapid spread of disease to a large number of people in a given population within a short period of time. For example, in meningococcal i...

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The rapid spread of disease to a large number of people in a given population within a short period of time. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic.

Example of an epidemic showing the number of new infections over time.

      Epidemics of infectious disease are generally caused by several factors including a change in the ecology of the host population (e.g., increased stress or increase in the density of a vector species), a genetic change in the pathogen reservoir or the introduction of an emerging pathogen to a host population (by movement of pathogen or host). Generally, an epidemic occurs when host immunity to either an established pathogen or newly emerging novel pathogen is suddenly reduced below that found in the endemic equilibrium and the transmission threshold is exceeded.

An epidemic may be restricted to one location; however, if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic.The declaration of an epidemic usually requires a good understanding of a baseline rate of incidence; epidemics for certain diseases, such as influenza, are defined as reaching some defined increase in incidence above this baseline.A few cases of a very rare disease may be classified as an epidemic, while many cases of a common disease (such as the common cold) would not. An epidemic can cause enormous damage through financial and economic losses in addition to impaired health and loss of life.

Causes -

There are several changes that may occur in an infectious agent that may trigger an epidemic. These include:

Increased virulence

Introduction into a novel setting

Changes in host susceptibility to the infectious agent

An epidemic disease is not required to be contagious, and the term has been applied to West Nile fever and the obesity epidemic (e.g., by the World Health Organization), among others.

The conditions which govern the outbreak of epidemics include infected food supplies such as contaminated drinking water and the migration of populations of certain animals, such as rats or mosquitoes, which can act as disease vectors.

Epidemics can be related to seasonality of certain infectious. Seasonality may enter into any of the eight key elements of the system: susceptible recruitment via reproduction,  transmission,  acquired immunity and recovery,  waning immunity, natural mortality,  symptomatology and pathology (which may be acute or chronic, depending on the disease),  disease-induced mortality, and  cross-species transmission. Influenza, the common cold, and other infections of the upper respiratory tract, such as sore throat, occur predominantly in the winter. There is another variation, both as regards the number of people affected and the number who die in successive epidemics: the severity of successive epidemics rises and falls over periods of five or ten years.