A pandemic, derived from the Greek "pan-demos" meaning 'all people,' is an infectious disease epidemic that rapidly spreads across multiple continents or worldwide, affecting a substantial portion of the human population. Unlike stable endemic diseases, pandemics are characterized by a sudden, often exponential, surge in cases, crossing international boundaries.

Throughout history, devastating examples include the 14th-century Black Death, which killed up to half of Europe's population, and the 1918 H1N1 influenza, known as the Spanish Flu, considered the deadliest pandemic ever. More recently, HIV/AIDS, the 2009 swine flu, and COVID-19 have underscored the ongoing global threat. In response, World Health Organization (WHO) member states are negotiating an International Treaty on Pandemic Prevention, Preparedness and Response, with a draft due by the 2024 World Health Assembly.