What Is It?

Encephalitis means inflammation of the brain. This inflammation usually is triggered by a viral infection. Encephalitis can kill, or it may produce only a short, mild illness.

Of the many different viruses that can cause encephalitis, enteroviruses (particularly coxsackievirus and echovirus) are the most common cause in the United States. Encephalitis also can be caused by the herpes simplex virus, which also causes cold sores and genital herpes. This type of encephalitis is less common but tends to be more severe. The mumps and measles viruses also can produce encephalitis. All of these viruses are transmitted directly from one person to another.

Other viruses that cause encephalitis are spread by animals. Equine viruses, which also are found in horses, and arboviruses are transmitted from animals to humans by the bite of insects, typically mosquitoes. The West Nile virus, one of the arboviruses, is widespread in Africa, Central Europe, the Middle East and Asia. Since 1999, it has become increasingly common in the United States. West Nile virus does not cause encephalitis in most humans who are infected, but when it does, the symptoms can be severe.


The symptoms of encephalitis range from mild to severe, and can be life-threatening. Fortunately, most cases are not severe. Possible symptom, beginning with the two most common, include the following:

  • Sudden fever
  • Headache
  • Vomiting
  • Stiff neck and back
  • Drowsiness
  • Extreme sensitivity to light
  • Seizures
  • Coma
  • Paralysis
  • Convulsions

Many of these symptoms also occur in other conditions that affect the brain, including migraine headaches and life-threatening conditions such as bleeding in the brain. When a person has a fever along with the other symptoms, some kind of infection is most likely.


If your doctor suspects you have encephalitis, he or she will order tests such as a computed tomography (CT) scan or magnetic resonance image (MRI) of the brain. A lumbar puncture (spinal tap) may be used to draw spinal fluid and test it to determine what virus is causing the encephalitis.

Expected Duration

Encephalitis can last from a few days to several months, depending on the virus involved and the severity of the individual case.


When outbreaks of insect-borne encephalitis occur, people in the affected communities should remove pools of standing water, where mosquitoes can breed, and should use insect repellant, particularly when outdoors. The most effective insect repellants contain the chemical called DEET. Travelers can be given vaccines against one cause, Japanese B encephalitis, which is common in Japan and other parts of Asia.


Antiviral drugs are available for several causes of encephalitis, although those for the herpes virus are the most effective. Treatment of symptoms — seizures, pain and fever — also may be helpful.

When To Call A Professional

Contact a doctor if someone in your family appears to be suddenly confused, is very hard to arouse, seems to have lost consciousness or has severe headaches. Whether these symptoms are caused by encephalitis or another condition, they require medical attention. In babies, a bulge in the fontanelle (the soft spot of the skull) is another important warning sign.


Encephalitis is most dangerous in babies and senior citizens, but it can be serious and even fatal in people of all ages. With severe cases, recovery is slow and may involve therapy to regain certain skills. The prospects for a complete recovery vary depending upon the type of virus involved. Encephalitis from the herpes virus can cause permanent injury. Eastern equine encephalitis is rare, with fewer than 10 cases per year in the United States. However, 50 percent to 60 percent of cases are fatal, and most survivors have permanent brain damage.

Johns Hopkins patient information

Last revised:

Diseases and Conditions Center

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.