The recent outbreak of the rare Andes hantavirus has once again drawn attention to terms that became widely familiar during the COVID‑19 pandemic: endemic, epidemic and pandemic.

These concepts are often grouped together or misused in public discourse, but in epidemiology they have precise meanings. Importantly, they describe how a disease spreads — not how dangerous it is.

A disease that occurs regularly in certain regions is called endemic. When a disease becomes endemic, the number of people falling ill remains relatively constant over time. A typical example is malaria, which annually affects 300 million people worldwide, with most cases in the tropics.

The spread of a disease is termed an epidemic when it occurs with unusual frequency in a certain region and for a limited period of time. Epidemics can occur when a pathogen mutates or when a disease is newly introduced into an area. An early example is smallpox, introduced to the Americas via Europeans, which killed up to 90% of the Indigenous population.

If a disease spreads across countries and continents, experts refer to it as a pandemic. Successful control depends on cooperation among health systems of different countries. Influenza repeatedly assumes pandemic proportions; the 1918 Spanish flu killed 25 million to 50 million people.

The terms epidemic and pandemic normally refer to infectious diseases, but they are sometimes used metaphorically for noncommunicable conditions like "diabetes epidemic" or "opioid epidemic." However, some note that misuse of the word can distract from perpetrator responsibility.

Source: www.dw.com