A pandemic (/pænˈdɛmɪk/ pan-DEM-ik) is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. Widespread endemic diseases with a stable number of infected individuals such as recurrences of seasonal influenza are generally excluded as they occur simultaneously in large regions of the globe rather than being spread worldwide.

Throughout human history, there have been a number of pandemics of diseases such as smallpox. The Black Death, caused by the Plague, caused the deaths of up to half of the population of Europe in the 14th century.^[2][3][4][5] The term pandemic had not been used then, but was used for later epidemics, including the 1918 H1N1 influenza A pandemic—more commonly known as the Spanish flu—which is the deadliest pandemic in history.^[6][7][8] The most recent pandemics include the HIV/AIDS pandemic,^[a][9] the 2009 swine flu pandemic and the COVID-19 pandemic. Almost all these diseases still circulate among humans though their impact now is often far less.

In response to the COVID-19 pandemic, 194 member states of the World Health Organization began negotiations on an International Treaty on Pandemic Prevention, Preparedness and Response, with a requirement to submit a draft of this treaty to the 77th World Health Assembly during its 2024 convention.^[10][11] Further, on 6 May 2024, the White House released an official policy to more safely manage medical research projects involving potentially hazardous pathogens, including viruses and bacteria, that may pose a risk of a pandemic.^[12][13]

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