A pandemic (from Greek πᾶν pan "all" and δῆμος demos "people") is a disease that has spread across a large region, for instance multiple continents, or worldwide. A widespread endemic disease with a stable number of infected people is not a pandemic.

Flu pandemics generally exclude recurrences of seasonal flu. 

Throughout history, there have been a number of pandemics of diseases such as smallpox and tuberculosis. One of the most devastating pandemics was the Black Death, which killed an estimated 75–200 million people in the 14th century. Current pandemics include HIV/AIDS and the 2019 coronavirus disease. Other notable pandemics include the 1918 influenza pandemic (Spanish flu) and the 2009 "swine" flu pandemic (H1N1).

Listen to Health Made Simple to learn why COVID-19 is different from the other 3 pandemics in the past 100 years.