Why do epidemic outbreaks occur?

A variety of factors may be involved in an infectious disease epidemic. For instance, a novel form of a pathogen may emerge, a form for which widespread immunity or resistance does not exist.

Pathogens can also mutate in such a way that previously effective medication become less effective or altogether ineffective in the management of the new variety of the disease. This is generally a significant factor in influenza epidemics.

Other factors that directly increase the spread of a given disease, such as contamination of drinking water sources by a waterborne pathogen such as cholera, can contribute to an epidemic outbreak.

Additionally, human behavior, such as unprotected sexual intercourse with multiple partners or the use of intravenous drugs via dirty needles, can be a significant factor in an infectious outbreak.