A disease outbreak that is widespread enough to affect people outside of a local or regional area is defined as an epidemic. The term is sometimes applied to noninfectious diseases, such as cancer or obesity, that have an impact on a large group of individuals in the same way that infectious disease does. This extension of the meaning of epidemic has led to the use of the word in a variety of other contexts, such as the “obesity epidemic” or “climate change epidemic.”
Epidemics can be classified in different ways depending on their characteristics. A common-source outbreak is one that spreads from a single source, such as an untreated sewage spill or a person who is infected with a virus that can be transmitted through close contact. A point-source epidemic spreads from a specific event, such as the leukemia cases in Hiroshima after the atomic bomb, or hepatitis A among patrons of a Pennsylvania restaurant who ate green onions. An epidemic that combines both common-source and propagated characteristics is considered a mixed epidemic.
A severe pandemic has the potential to divide societies, as was seen during premodern outbreaks of yellow fever (in America), cholera in India, and the 1918 flu epidemic (in the US). Severe pandemics can also be costly due to hospitalization costs and productivity losses from social distancing and death. In addition, pandemic severity itself can impact these costs; high-severity pandemics typically have higher overall economic impacts than lower-severity pandemics.