Rabies is a viral disease that affects the central nervous system (CNS) of mammals and has an extremely high case fatality rate. Once clinical signs develop, there are very few survivors. Vaccines can protect pets, as well as people exposed to these animals, but the maintenance of rabies viruses in wildlife complicates control. In humans, illness can be prevented by administering anti-rabies antibodies and a series of vaccinations, provided exposure is recognized before the symptoms appear. However, people in impoverished countries do not always have access to effective post-exposure prophylaxis. Due to this and other factors, such as inadequate levels of vaccination in dogs and cats, the annual incidence of human rabies is estimated to be 40,000 or more cases, worldwide. A few cases occur even in nations with good medical care, typically in people who did not realize they were exposed.

Closely related lyssaviruses circulate among bats in the Eastern Hemisphere, and can cause an illness identical to rabies in people and domesticated animals. Rabies vaccines and post-exposure prophylaxis are thought to provide some protection against some of these viruses, but not others. Rabies-related lyssaviruses can be found even in countries classified as rabies-free.