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What Is Encephalitis?

Encephalitis is an inflammation (swelling and irritation) of the brain. In most cases, a virus causes this inflammation.

Encephalitis is also called acute viral encephalitis or aseptic encephalitis.

Who Gets Encephalitis?

Encephalitis (pronounced: in-seh-fuh-LYE-tus) is a rare disease. Most cases happen in children, the elderly, and people with a weakened immune system (from HIV/AIDS, cancer, etc.).

Several thousand cases of encephalitis are reported to the Centers for Disease Control and Prevention (CDC) every year. But health experts think that many more cases happen that aren't reported because symptoms vary and can be mild.

What Are the Signs & Symptoms of Encephalitis?

Symptoms in mild cases of encephalitis usually include:

  • fever
  • headache
  • poor appetite
  • loss of energy
  • a general sick feeling

Serious cases of encephalitis can cause:

  • a high fever
  • severe headache
  • nausea and vomiting
  • stiff neck
  • confusion
  • personality changes
  • convulsions (seizures)
  • problems with speech or hearing
  • hallucinations
  • memory loss
  • drowsiness
  • coma

Because encephalitis can happen during or after common viral illnesses, symptoms of these illnesses can start before encephalitis happens. But often, it appears without warning.

What Causes Encephalitis?

Three groups of viruses are common causes of encephalitis:

  1. Herpes viruses, such as chickenpox, EBV (Epstein-Barr virus, which causes mono), and herpes simplex (which causes cold sores).
  2. Viruses and other germs transmitted by insects, like West Nile virus (spread through a mosquito bite) and the germs that cause Lyme disease and Rocky Mountain spotted fever (spread through tick bites).
  3. Viruses that cause once-common childhood infections, such as measles, mumps, and German measles. Thanks to immunizations, it's rare today for someone to develop encephalitis from these illnesses.

Less often, encephalitis can be:

  • caused by an infection from bacteria , such as bacterial meningitis
  • a complication of other infectious diseases like syphilis
  • due to a parasite, like toxoplasmosis (found in infected cat feces) in people with weakened immune systems

In many cases, the cause of encephalitis isn't known.

Is Encephalitis Contagious?

Brain inflammation itself is not contagious. But the viruses that cause encephalitis can be. Of course, getting a virus does not mean that someone will develop encephalitis.

How Is Encephalitis Diagnosed?

Doctors use several tests to diagnose encephalitis, including:

  • imaging tests, such as computed tomography (CT) scans or magnetic resonance imaging (MRI), to check the brain for swelling, bleeding, or other problems
  • electroencephalogram (EEG), which records the electrical signals in the brain, to check for unusual brain waves
  • blood tests to look for bacteria or viruses in the blood. These also can show if the body is making antibodies (specific proteins that fight infection) in response to a germ
  • lumbar puncture, or spinal tap, which checks cerebrospinal fluid (the fluid that surrounds the brain and spinal cord) for signs of infection

How Is Encephalitis Treated?

Most people with encephalitis need care in a hospital, usually in an intensive care unit (ICU). Doctors will watch their blood pressure, heart rate, breathing, and body fluids to prevent further swelling of the brain.


  • Antiviral drugs can treat some forms of encephalitis, such as the type caused by the herpes simplex virus.
  • Corticosteroids may be used to reduce brain swelling.
  • Anticonvulsants might be given to someone having seizures.
  • Medicines you can buy without a prescription, like acetaminophen, can help with fever and headaches.
  • Antibiotics don't work against viruses, so aren't used to treat most forms of encephalitis.

Many people with encephalitis make a full recovery. In some cases, brain swelling can cause lasting problem like learning disabilities, speech problems, hearing loss, memory loss, or lack of muscle control. Speech therapy, physical therapy, or occupational therapy can help in these cases.

How Long Does Encephalitis Last?

Most of the time, the acute phase of the illness (when symptoms are the most severe) lasts up to a week. Full recovery can take longer, often several weeks or months.

Can Encephalitis Be Prevented?

The best way to prevent encephalitis is to avoid getting infected with the viruses or other germs that can cause it. Regular hand washing will help limit the spread of some of these germs. Staying as healthy as possible by eating a balanced diet and getting plenty of rest can help keep your immune system in shape. Vaccines are also an important way to protect people from diseases like chickenpox and measles.

In areas where viruses and other germs spread by insect bites, protect yourself by wearing long sleeves and pants and applying an insect repellent. Also, try to avoid unnecessary outdoor activities at dawn and dusk when mosquitoes are most likely to bite.

When Should I Call the Doctor?

Get medical care right away if you have any of these symptoms:

  • a severe headache
  • convulsions (seizures)
  • stiff neck
  • can't look at bright lights
  • double vision
  • trouble walking
  • problems with speech or hearing
  • trouble moving an arm or leg
  • loss of sensation anywhere in the body
  • sudden personality changes
  • problems with memory
  • drowsiness or loss of energy
  • loss of consciousness (passing out)

Reviewed by: Yamini Durani, MD
Date Reviewed: Sep 1, 2023

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