When diseases occur in a human population at higher than expected levels, those diseases are said to be "epidemic." Animals also experience outbreaks at levels greater than expected, and when that happens, it is called "epizootic". There have been several very large epizootics in recent years, with one of the biggest being the West Nile virus expansion into the Americas. This caused significant bird mortality, and it expanded into the human population as well. The effect of epizootics on human health is, therefore, of great importance. Still, epizootics can be extremely important even if humans are unaffected. This book describes several recent epizootics, discusses why they occurred, and what kind of response may be necessary.