Herpes simplex virus is the most common cause of sporadic fatal encephalitis. It mainly involves individuals more than 40 years of age. Primary HSV 1 often present as stomatitis and HSV type 2 manifest as venereally transmitted genital eruption. These virus then migrates along nerve axons to sensory ganglia, and persists in a latent form and may be reactivated later. In adults encephalitis is caused by HSV type 1. In neonates HSV encephalitis is caused by type 2 virus during passage through the bih canal of a mother with active genital lesions. HSV type 2 in adults causes meningitis, rather than encephalitis. Ref: Clinical Neurology By Roger P. Simon, 7th Edition, Chapter 1