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)