An outbreak is a sudden increase in the number of cases of a disease, typically an infectious disease, above what is normally expected in the population. Outbreaks can occur in any size community, from a city to the world. They are usually caused by introduction of a new disease into a community or by re-emergence of a previously controlled or dormant disease. An outbreak may be restricted to one particular age group or geographic area.
Outbreaks usually follow one or more of three patterns: exponential, where each infected person infects two or more other people; linear, where each infected person infects only one other person; and plateau, where the number of new cases levels off even though some people are still susceptible to infection. The most common cause of an outbreak is failure to control an infectious disease through vaccination or treatment. Other causes include natural disasters such as floods and earthquakes that damage public health infrastructure and disrupt access to medical care; war and civil unrest that also damage public health infrastructure and disrupt access to medical care; and deliberate release of pathogens (biological warfare).
The ability to detect outbreaks early is essential for effective control measures to be put in place. Early detection may be achieved through surveillance systems that monitor diseases activity within communities on a regular basis. These systems can be enhanced by alerting mechanisms that rapidly disseminate information about increases in disease activity so that action can be taken promptly. In many countries, national Public Health Institutes play an important role in detecting outbreaks and responding to them effectively.