Encephalitis is an inflammation (swelling and irritation) of the brain that is usually the result of a viral infection. If not treated immediately, encephalitis can alter brain function and become life-threatening.

The most common symptoms of encephalitis are fever, severe headache, and confusion. Other symptoms may develop, such as sensitivity to light, nausea and vomiting, stiff neck and back, and drowsiness. Sometimes severe symptoms develop, such as seizures, tremors, personality changes, and even coma. In general, symptoms that develop suddenly and are serious from the start usually mean a more severe, life-threatening form of encephalitis.

Encephalitis is most often caused by a virus, such as the virus that causes cold sores and genital herpes (herpes simplex), mumps, measles, chickenpox, mononucleosis (Epstein-Barr virus), influenza, or German measles (rubella). Although very rare in the United States, encephalitis may be spread by infected mosquitoes and ticks.

Treatment usually includes hospitalization and use of the antiviral medicine acyclovir along with supportive care for symptoms.