adminA disease outbreak occurs when a number of cases of a contagious illness exceeds what would be expected in a defined community or geographic area. The Centers for Disease Control and Prevention(link is external and opens in a new window) defines an epidemic as a “rapid increase of disease cases above what can be explained by local factors.” Yellow fever, smallpox, and measles are prime examples of epidemic diseases. Other diseases—such as obesity and the spread of HIV/AIDS—are also considered epidemics.
The emergence of new infectious disease threats, and the re-emergence of old ones, illustrates the fact that global health systems are woefully unprepared to deal with large epidemics, and that these outbreaks can have enormous social and economic consequences. Disease outbreaks are caused by microbes that can spread directly from person to person, or from animal to human and then to other humans, via insect vectors such as mosquitoes or rodents. The emergence and spread of these disease agents often has complex causes.
Environmental conditions—such as water supply, food, air quality and sanitation facilities—are essential to slowing the spread of disease in an outbreak. Microbes are constantly adapting to their environment, and these adaptations may enable them to evade the immune system or gain new abilities to infect other cells. These changes can also make vaccines less effective. In addition, people’s behavior—including vaccination status, travel habits and how they handle animals and the environment—can affect the risk of developing an epidemic.
