Encephalitis is inflammation of the brain, almost always caused with a virus; often there is an associated meningitis. It is a rare complication of certain common diseases (e.g., mumps, Herpes simplex virus) and a specific manifestation of less common viruses, often carried by insects. Typically an acute illness with headache and fever, it may lead to evidence of patchy inflammation of brain tissue, such as personality change, epilepsy, localized weakness, or rigidity. It may progress to impairment of consciousness and coma. A particular type, Encephalitis lethargica, occurred as an epidemic, early in the 20th century leading to a chronic disease resembling Parkinson's disease but often with permanent mental changes.

Encephalitis is diagnosed by tests on cerebrospinal fluid, which is obtained by lumbar puncture, and possibly brain scans. The anti-viral drug acyclovir is effective against H. simplex encephalitis, otherwise treatment is mainly supportive.