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).


Nucleus factsheet image

Copyright © Nucleus Medical Media, Inc.


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:

  • Viruses carried by mosquitoes such as West Nile or Eastern equine encephalitis
  • Herpes simplex virus (HSV)
  • Chickenpox virus
  • Enterovirus
  • Influenza virus
  • Epstein-Barr virus (EBV)
  • Polio virus
  • Measles virus
  • Mumps virus
  • Rotavirus
  • Rabies
  • 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


    The symptoms may range from mild to severe. Severe symptoms can include permanent neurological damage. Encephalitis can also lead to death.

    Milder symptoms include:

  • Fever
  • Weakness, severe fatigue
  • Headache
  • Sensitivity to light
  • Stiff neck and back
  • Vomiting
  • Muscle aches
  • Rash
  • Yawning
  • More severe symptoms may include:

  • Changes in consciousness
  • Personality changes
  • Confusion
  • Irritability
  • Seizures
  • Loss of mobility
  • Progressive drowsiness
  • Trouble walking
  • Trouble speaking
  • Trouble swallowing
  • Diagnosis

    You will be asked about your symptoms and medical history. A physical exam will be done.

    Your bodily fluids may be tested. This can be done with:

  • Blood tests
  • Lumbar puncture to test the cerebrospinal fluid that surrounds the brain and spinal cord
  • Brain biopsy
  • Images may be taken of your head. This can be done with:

  • CT scan
  • MRI scan
  • Your brain's electrical activity may be tested. This can be done with an electroencephalogram (EEG).


    Treatment is mostly supportive. It may include:

  • Antiviral drugs to shorten the duration of the illness
  • Steroids to reduce brain inflammation
  • Diuretics to decrease elevated intracranial pressure
  • Intubation with hyperventilation to decrease elevated intracranial pressure, and to maintain respiration and ventilation
  • Anticonvulsants to prevent and/or treat seizures
  • Prevention

    To help reduce your chance of encephalitis:

  • Make sure that you and your children are vaccinated against preventable viral illnesses
  • Protect yourself from mosquito bites:     
  • Fix window screens.
  • Drain standing water around your home.
  • Wear long clothes after dark.
  • Use repellent when you are outside.
  • Use proper mosquito netting at night. Look for netting treated with insecticide.