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
- Changes in host exposure 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, among others.
The conditions which govern the outbreak of epidemics include infected food supplies, such as drinking water contaminated by waste from people with CHOLERA or typhoid fever (see ENTERIC FEVER); milk infected with TUBERCLE bacillus; or ‘fast food’ products contaminated with salmonella. The migrations of certain animals, such as rats, are in some cases responsible for the spread of PLAGUE, from which these animals die in great numbers. Certain epidemics occur at certain seasons: for example, whooping-cough occurs in spring, whereas measles produces two epidemics - as a rule, one in winter and one in March. 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 persons affected and the number who die in successive epidemics: the severity of successive epidemics rises and falls over periods of five or ten years.
Read more about this topic: Epidemic