Rabies is a viral disease of mammals most often transmitted through the bite of a rabid animal. Rabies is almost invariably fatal if post-exposure prophylaxis is not administered prior to the onset of symptoms. It is a significant killer of livestock in some countries, causing inflammation of the brain. The incubation period of the disease varies, usually taking a few months. However, when the infection reaches the central nervous system the untreated infection is usually fatal within days.