Rabies is a viral neuroinvasive disease that causes acute encephalitis (inflammation of the brain) in warm-blooded animals. It is zoonotic (i.e., transmitted by animals), most commonly by a bite from an infected animal but occasionally by other forms of contact. Rabies is almost invariably fatal if post-exposure prophylaxis is not administered prior to the onset of severe symptoms. It is a significant killer of livestock in some countries.
The rabies virus travels to the brain by following the peripheral nerves. The incubation period of the disease depends on how far the virus must travel to reach the central nervous system, usually taking a few months. Once the infection reaches the central nervous system and symptoms begin to show, the infection is practically untreatable and usually fatal within days.
Early-stage symptoms of rabies are malaise, headache and fever, later progressing to more serious ones, including acute pain, violent movements, uncontrolled excitement, depression and inability to swallow water. Finally, the patient may experience periods of mania and lethargy, followed by coma. The primary cause of death is usually respiratory insufficiency.
The term is derived from the Latin ''rabies'', "madness". This, in turn, may be related to the Sanskrit ''rabhas'', "to do violence". The Greeks derived the word "lyssa", from "lud" or "violent"; this root is used in the name of the genus of rabies ''lyssavirus''.
In unvaccinated humans, rabies is almost always fatal after neurological symptoms have developed, but prompt post-exposure vaccination may prevent the virus from progressing. Rabies kills around 55,000 people a year, mostly in Asia and Africa. There are only six known cases of a person surviving symptomatic rabies, and only three known cases of survival in which the patient received no rabies-specific treatment either before or after illness onset.
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