The world’s governments struggle to contain outbreaks of infectious disease. In fact, some diseases become pandemics, which threaten the global economy and social stability.
An epidemic is a sudden increase in the number of cases of a disease more than normal expectancy in a community or geographical area. A disease must spread from person to person for it to be classified as an epidemic. The most well-known examples of epidemics are the cholera, smallpox and SARS (Severe Acute Respiratory Syndrome) outbreaks that occurred in the late 20th century.
Some epidemics, like the Asian flu in 1957-1958, are a mix of both common-source and propagated outbreak characteristics. For example, people who attended a music festival and contracted shigellosis from contaminated food spread the disease to others, thus becoming part of a common-source outbreak. Other epidemics are zoonotic, caused by animals carrying disease that then transmit to humans. For example, the Nipah and Hendra viruses are zoonotic ebolaviruses that can cause severe respiratory illness.
Epidemics can be catalyzed by a wide range of factors, including the weather, seasonality or geographic location. Environmental factors such as droughts or flooding may make it easier for disease to travel. Certain types of pathogens are more likely to cause an outbreak than others. For example, filoviruses (including Ebola and Marburg viruses) and paramyxoviruses (including rabies and plague) are more likely to cause high-mortality outbreaks than hepatitis C and influenza viruses.
A new disease may also be a biological disaster, created by a mutation or by exposure to chemical and radioactive hazards. Biological disasters can be as destructive as natural disasters, but they can often be more difficult to predict and respond to because the source of the pathogen is unknown.