What is the term for the spread of a disease across a large area?

The term that describes the spread of a disease across a large area is "epidemic." An epidemic occurs when there is a sudden increase in the number of cases of a disease above what is normally expected in a particular geographic area. This situation often indicates that the disease is not confined to a single community or location, but rather affects a wider region or population.

In contrast, an outbreak typically refers to a more localized occurrence of disease, and contagion describes the process of disease transmission from one individual to another. An endemic is a disease or condition regularly found and consistently stable within a particular geographic area or population. Thus, the term "epidemic" specifically captures the significant escalation and widespread nature of a disease, making it the correct choice.