The exact cause of encephalitis is often unknown, but the most commonly diagnosed cause is a viral infection. Bacterial infections and noninfectious inflammatory conditions also may cause encephalitis.

An infection may result in one of two conditions affecting the brain:

  • Primary encephalitis occurs when a virus or other infectious agent directly infects the brain. The infection may be concentrated in one area or widespread. A primary infection may be a reactivation of a virus that had been inactive (latent) after a previous illness.
  • Secondary (postinfectious) encephalitis is a faulty immune system reaction in response to an infection elsewhere in the body. Instead of solely attacking the cells causing an infection, the immune system also mistakenly attacks healthy cells in the brain.

    Secondary encephalitis often occurs two to three weeks after the initial infection. Rarely, secondary encephalitis occurs as a complication of a live virus vaccination.

Common viral causes

Common causes of encephalitis include:

  • Herpes simplex virus. There are two types of herpes simplex virus (HSV). Either type can cause encephalitis. HSV type 1 (HSV-1) is usually responsible for cold sores or fever blisters around your mouth, and HSV type 2 (HSV-2) commonly causes genital herpes. Encephalitis caused by HSV-1 is rare, but it has the potential to cause significant brain damage or death.
  • Other herpes viruses. Other herpes viruses that may cause encephalitis include the Epstein-Barr virus, which commonly causes infectious mononucleosis, and the varicella-zoster virus, which commonly causes chickenpox and shingles.
  • Enteroviruses. These viruses include the poliovirus and the coxsackievirus, which usually cause an illness with flu-like symptoms, eye inflammation and abdominal pain.
  • Mosquito-borne viruses. Arboviruses, or arthropod-borne viruses, are transmitted by mosquitoes or other blood-sucking insects. Mosquito-borne viruses can cause infections that include West Nile, La Crosse, St. Louis, western equine and eastern equine encephalitis.

    Mosquitoes transfer the virus from a nonhuman host — such as a bird, chipmunk or horse — to humans. Symptoms of an infection may appear within a few days to a couple of weeks after exposure to an arbovirus.

  • Tick-borne viruses. The Powassan virus is a well-known tick-transmitted virus that causes encephalitis in the U.S. and Canada. Symptoms usually appear about a week after exposure to the virus.
  • Rabies virus. Infection with the rabies virus, which is usually transmitted by a bite from an infected animal, causes a rapid progression to encephalitis once symptoms begin. Rabies is a rare cause of encephalitis in the U.S.
  • Childhood infections. Common childhood infections — such as measles (rubeola), mumps and German measles (rubella) — used to be fairly common causes of secondary encephalitis. These causes are now rare because of the availability of vaccinations for these diseases.
May. 15, 2014

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