What term is used to describe a sudden outbreak of disease affecting a large number of animals?

The term used to describe a sudden outbreak of disease affecting a large number of animals is "epizootic." This term specifically refers to outbreaks that occur in animal populations, distinguishing it from "epidemic," which typically refers to human populations. An epizootic indicates that the disease has rapidly spread across a significant geographic area or population of animals.

In contrast, endemic denotes a disease that is regularly found and consistently present within a particular geographic area or population. Pandemic refers to an epidemic that has spread over multiple countries or continents, usually affecting humans. While a pandemic can include animal diseases as well, epizootic is the more precise term for outbreaks confined to animal populations, making it the correct choice in this context.