An epidemic is defined as an outbreak of a contagious disease that spreads rapidly and widely. What is the difference between an epidemic and a pandemic, and how should the 1918-1919 Spanish flu be classified?

A pandemic is an epidemic of 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. Widespread endemic diseases with a stable number of infected people such as recurrences of seasonal flu are generally excluded as they occur simultaneously in large regions of the globe rather than being spread worldwide.

The Spanish flu, also known as the 1918 flu pandemic, was an unusually deadly influenza pandemic. Lasting from January 1918 to December 1920, it infected 500 million people - about a third of the world's population at the time. The death toll is estimated to have been anywhere from 17 million to 50 million, and possibly as high as 100 million, making it one of the deadliest pandemics in human history.