Epidemiology 101: Defining endemic, epidemic, and pandemic disease spread

This explainer distinguishes three epidemiological classifications often confused in public discourse. **Endemic** diseases occur regularly in specific regions with relatively constant case numbers—malaria exemplifies this, affecting ~300 million annually in tropical regions. **Epidemic** describes when cases in a region exceed the normally expected (endemic) level, often triggered by pathogen mutation, increased transmissibility, or introduction to a naive population (e.g., European smallpox in the Americas, killing ~90% of indigenous populations). **Pandemic** indicates global spread across countries and continents, dependent on international health cooperation; pandemics typically involve novel pathogens causing high absolute case numbers due to population-wide susceptibility. Importantly, these terms characterize *distribution patterns*, not inherent danger—endemic diseases can be fatal, while pandemics may cause relatively mild illness. The article notes these classifications apply primarily to infectious diseases, though terms like "opioid epidemic" are metaphorical extensions.