Encephalitis.

Encephalitis is a serious and potentially treatable infection of the central nervous system. A pathogen is identified in less than 50% of cases. The differential diagnosis includes acute infection, immune-mediated causes, and other central nervous system processes. Emergent investigations include blood work, cerebrospinal fluid analysis, and neuroimaging. Empiric acyclovir and antibiotics should be started immediately to maximize the child's chance of neurologic recovery.