The most common cause of sporadic viral encephalitis is :
“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
Japanese B encephalitis
Herpes simplex encephalitis
HIV encephalitis
Rubeola encephalitis