Which of the folloing virus is the commonest cause of sporadic encephalitis?

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