Encephalitis is swelling of the brain. The swelling may involve the whole brain, or just parts of the brain. Encephalitis may just occur in individuals (sporadic) or may affect many people in a particular area (epidemic).

Encephalitis
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Encephalitis is most often caused by a viral infection. In the United States, the most common cause of sporadic encephalitis is the herpes simplex virus (HSV). Epidemic causes of encephalitis are usually mosquito or tick-borne viruses.

The most common viruses that cause encephalitis include:

Not all encephalitis is caused by a virus. Some may be due to an overreaction of the immune system.