A 39-year-old man with acute mental status changes.

The incidence of herpes simplex virus (HSV) encephalitis exceeds that of other forms of acute viral encephalitis. It is widely acknowledged that the use of PCR amplification for the detection of HSV DNA in cerebrospinal fluid (CSF) has increased our diagnostic sensitivity for this illness, leading to the diagnosis of HSV encephalitis in a wide range of clinical presentations. It is important to distinguish HSV from other viral encephalitides because it is the only form of viral encephalitis for which effective treatment exists. Prognosis is greatly improved with early treatment.