Herpes Simplex Encephalitis is the most common cause of sporadic encephalitis in western countries. In the general population 1/250,000 to 1/500,000 of the population is newly affected each year. The disease is charaterized by hemorrhagic necrosis of the temporal lobe. Even with appropriate antiviral treatment, mortality and morbidity are high. The mortality rate at present is roughly 30% after 1 year of treatment. Of those who survive, 50% have moderate to severe neurologic impairment.
Most cases are caused by Herpes Simplex Virus-1 (HSV-1). The actual pathogenesis of herpes simplex encephalitis is unknown. However, it has been speculated that primary or recurrent virus reaches the temporal lobe by ascending neural pathways.