These infections may be Epidemic (eg, due to arbovirus, echovirus, coxsackievirus, or poliovirus [in some underdeveloped countries]) Sporadic (eg, due to cytomegalovirus or to herpes simplex, varicella-zoster, lymphocytic choriomeningitis, rabies, or mumps virus) Mosquito-borne arboviral encephalitides infect people during the spring, summer, and early fall when the weather is warm (see table Some Arboviral Encephalitides).