encephalitis (ĕnsĕf´əlī´təs), general term used to describe a diffuse inflammation of the brain and spinal cord, usually of viral origin, often transmitted by mosquitoes, in contrast to a bacterial infection of the meninges (membrane surrounding the brain and spinal cord), known as meningitis. Diagnostic symptoms include capillary congestion, small hemorrhages into perivascular spaces, accumulation of plasma cells and lymphocytes, and increased pressure and protein content of cerebrospinal fluid.
Among the several forms of viral brain inflammation are rabies, polio, and two types transmitted by the mosquito: equine encephalitis in its various forms and St. Louis encephalitis. The latter two have appeared in epidemic form in the United States and are characterized by high fever, prolonged coma (which is responsible for the disease being known as a "sleeping sickness" ), and convulsions sometimes followed by death. Encephalitis that results as a complication of another systemic infection is known as parainfectious encephalitis and can follow such diseases as measles (rubeola), influenza, and scarlet fever. The AIDS virus also infects the brain and produces dementia in a predictably progressive pattern. Although no specific treatment can destroy the virus once the disease has become established, many types of encephalitis can be prevented by immunization.