Epidemic
Definition:
An epidemic is defined as the occurrence in a community or region of cases of an illness or other health-related event clearly in excess of normal expectancy. (Park's textbook of preventive and social medicine)

Epidemic occur in same area in a specified population at the same season of year.



Types Of Epidemic:

A. Common Source epidemic.
 As the name suggest "The origin of epidemic is a common source". For example, water poisoning. Only those People who drink infected water from a particular water supply will develop the disease. Hence, all patient would have a common source.
Common source epidemic is further divided into two types:
a) Point source epidemic:
It is also known as "single exposure epidemic". In this case the time of exposure is brief and essentially simultaneous and all the resultant cases develop the disease within the incubation period of disease at the same time.
Epidemic curve in this case shows a sharp peak and sharp decline.
point-source-epidemic-curve-diagram

b) Common source, continuous or repeated exposure.
In this case epidemic reaches a sharp peak, but tails off gradually over a longer period of time although the sharp peak is attained after a longer time than a point source epidemic.
This occur because of prolonged and repeated exposures or when the source of infection is not eliminated.
For-example a prostitute may be a common source in a gonorrhoea outbreak but in this case epidemic curve will rise gradually as she infects her clients over a period of time. But in case of water borne cholera the epidemic curve reaches a sharp peak but tails of gradually. These are the two different models of common source continuous epidemic.

B. Propagated Epidemic.
A propagated epidemic occurs due to transmission of infections agent either

  • Person to Person
  • Through Vectors (insects etc)
  • Through animal reservoir
An infected person can transfer the infection to other healthy person and make him infectious as well and then he can further transmit the infection to to other persons by various means (direct contact, sexual contact, through vectors, etc).
The disease remain in the community until susceptible and unimmunized individuals are present.
The epidemic start to terminates when susceptible individuals decrease in number or the people develop immunity against the disease. Herd immunity plays special role in this case.
The epidemic curve shows gradual rise and tails of over a much longer time as compared to common source repeated exposure epidemic.

propagated-epidemic-curve
 
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