The most common cause of sporadic viral encephalitis is :: (A) Japanese B encephalitis, (B) Herpes simplex encephalitis, (C) HIV encephalitis, (D) Rubeola encephalitis
Answer is B. “The most common virus causing sporadic cases of encephalitis in immunocompetant adult are HSV-1, VZV, EBV and less commonly enterovirus.”
Remember: Epidemic of encephalitis are caused by Arbovirus which include:

Alphaviruses
Toga viruses
Bunyaviruses