Pandemic

A pandemic is an epidemic on a worldwide scale: a disease that has spread beyond one region to several countries or continents and usually affects very large numbers of people. The word describes how far a disease has spread, not how deadly it is, so a pandemic can be mild for most of those infected and still overwhelm health systems simply through the number of cases.

Pandemics most often begin when a new pathogen appears to which almost nobody has immunity, frequently a virus that has jumped from animals to people. If it passes easily from person to person, travel and trade carry it around the globe faster than vaccines or treatments can be developed. New strains of influenza are a classic cause, and HIV/AIDS and COVID-19 are more recent examples.

Students meet the term in epidemiology and virology, where it sits at the far end of a scale of disease occurrence. That scale runs from sporadic cases, which appear only occasionally, through endemic disease, which is constantly present in an area, and epidemics, which exceed the expected level, to pandemics.