Encephalitis, an inflammation of the brain parenchyma, presents as diffuse and-or focal neuropsychological dysfunction. Although it primarily involves the brain, the meninges are frequently involved (meningoencephalitis). Serious, complex, and potentially fatal disorder with noninfectious and infectious causes. Presents with acute onset of a febrile illness and altered mental status; typical features include headache, seizures, and focal neurologic signs. Acyclovir should be given in all cases of suspected viral encephalitis. Investigations should include blood cultures, neuroimaging (preferably MRI), and CSF analysis. Complications include seizures, hydrocephalus, and neurologic sequelae (e.g., behavioral disturbances, neuropsychiatric problems, and motor problems).