[Viral encephalitis].

Viral infections of the nervous system make up a wide range of disorders with a mainly benign outcome. However, in some cases there is severe, morbimortality. In viral encephalitis there is direct involvement of the brain parenchyma which is seen clinically as reduced consciousness, convulsions and/or focal neurological deficit. The especial attraction of some viruses for particular cells or structures determines the variety of clinical findings. The incidence and frequency of the various agents depends on several factors (geographical location of a certain virus, age and general health of the population concerned, etc.). In areas free of arbovirus the commonest aetiologies are; varicella, herpes simplex, parotiditis and enterovirus. Modern treatment (transplants, chemotherapy) of previously fatal diseases and the AIDS epidemic have increased the number of immunodeficient patients; the population is susceptible to viral infections of the nervous system which are infrequent (e.g. cytomegalovirus, papovavirus) or which follow a different course (e.g. measles, enteroviruses) to that in immunocompetent patients. Specific conditions are reviewed. Improvement in the general health and sanitation of the population, and the universal use and development of new vaccines will significantly reduce the incidence of viral encephalitis. Improved prognosis will be related to the use of modern laboratory techniques which permit early, sensitive, specific diagnosis and the development of antiviral agents.