A disease that affects many animals at the same time; an epidemic among animals.
This almost sounds like a made-up word, but it's been in use since the early 1700s. It's still in use today, at least in scientific and medical circles. From the French épizootie, which was formed from the Greek epi (among, upon) + zoon (animal). 'Epidemy' originates from demos (people, district) and was in use as a noun from around the same time.
The word caught my eye in a discussion of stage star Mary Anderson's 1876 tour of San Francisco, where some considered her just another empty-headed debutante: "'We have some dozen or two in this little city alone,' wrote an editor in the San Francisco Call, 'and the dramatic fever is becoming as universal and epidemic as the epizooty among horses a season or two ago.'"
(from Gilded Girls: Women Entertainers of the Old West, by JoAnn Chartier and Chris Enss)
"The control of the influenza is never a local question, but too many countries do not yet adopt the recommendations of international organizations, and sometimes several months are necessary before the emergence of an eipzooty alerts political attention...."
- Encyclopedia of Infectious Diseases: Modern Methodologies; Michel Tibayrenc, ed. (2007)