Nervous System Home > Encephalitis

Caused by bacterial and viral infections, encephalitis is a condition where there is an inflammation of the brain. Symptoms can include sudden fever, headache, and vomiting. In extreme cases, complications can include brain damage, seizures, and death. While anyone can have it, people with weakened immune systems are at the highest risk of contracting encephalitis.

What Is Encephalitis?

Encephalitis means inflammation of the brain. It can be caused by bacterial infections and, most often, viral infections.
Several thousand cases of encephalitis are reported each year, but many more may actually occur, since the symptoms may be mild to non-existent in most patients.

Types of Encephalitis

There are two types of the condition:
  • Primary encephalitis (also called acute viral encephalitis) is caused by a direct viral infection of the spinal cord and brain. The infection may be focal (located in only one area) or diffuse (located in many different areas).
  • Secondary encephalitis (also known as post-infective encephalitis) can result from complications of a current viral infection. When this type results from an immunization or earlier viral infection, it is known as acute disseminated encephalitis. This illness often occurs two to three weeks following the initial infection.


Most cases of encephalitis in the United States are caused by:
  • Enteroviruses
  • Herpes simplex virus types 1 and 2
  • A bite from a rabid animal (rabies virus)
  • Arboviruses, which are transmitted from infected animals to humans through the bite of an infected tick, mosquito, or other blood-sucking insect.
Lyme disease, a bacterial infection spread by tick bite, can also cause it.
(Click Cause of Encephalitis for more information.)
Written by/reviewed by:
Last reviewed by: Arthur Schoenstadt, MD
Last updated/reviewed:
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