Which term describes a disease occurrence affecting a large population across multiple countries?

The term that describes a disease occurrence affecting a large population across multiple countries is "pandemic." This classification is used when a new disease spreads easily from person to person and has sustained human-to-human transmission across different geographical regions. A pandemic implies that the disease's impact reaches a global scale, affecting numerous countries and regions simultaneously.

In contrast, an epidemic typically refers to an increase in the number of cases of a disease in a specific area or population that exceeds what is normally expected. An outbreak is similar but can be more localized than an epidemic and may involve a smaller number of cases. Endemic describes a disease or condition that is consistently present within a specific geographic area or population, maintaining a steady state without major fluctuations.

Recognizing these distinctions highlights why the term "pandemic" accurately characterizes the widespread and international nature of certain diseases, especially those that have transitioned from localized outbreaks to global health concerns.