Encephalitis is inflammation of the brain. The inflammation 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).
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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.

Risk Factors

Factors that may increase your chance of encephalitis include:
  • Living, working, or playing in an area where mosquito- or tick-borne viruses are common.
  • Not being immunized against diseases, such as:
    • Measles
    • Mumps
    • Chickenpox
    • Polio
    • Rotavirus
  • A suppressed immune system caused by certain medications, or health conditions, such as HIV infection
  • Having cancer—sometimes immune system overactivity may be the first sign of cancer
Newborns of mothers who have genital herpes are at risk for herpes simplex encephalitis

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