An acute (or rarely chronic) inflammatory process of the brain caused by SIMPLEXVIRUS infections which may be fatal. The majority of infections are caused by human herpesvirus 1 (HERPESVIRUS 1, HUMAN) and less often by human herpesvirus 2 (HERPESVIRUS 2, HUMAN). Clinical manifestations include FEVER; HEADACHE; SEIZURES; HALLUCINATIONS; behavioral alterations; APHASIA; hemiparesis; and COMA. Pathologically, the condition is marked by a hemorrhagic necrosis involving the medial and inferior TEMPORAL LOBE and orbital regions of the FRONTAL LOBE. (From Adams et al., Principles of Neurology, 6th ed, pp751-4).