Acute Encephalitis Syndrome (AES) is a group of Clinical neurologic manifestation caused by wide range of viruses, bacteria, fungus, parasites, spirochetes, chemical and toxins. The most common causes of acute viral encephalitis are Japanese encephalitis virus, West Nile virus (WNV), Eastern equine encephalitis virus (EEEV), Western equine encephalitis virus (WEEV), Venezuelan equine encephalitis virus (VEEV), Hendra virus (HeV), enteroviruses (ENV), Chandipura virus (CHPV), Nipah (NiV), Kyasanur forest disease (KFD), St. Louis encephalitis virus, Herpes simplex, poliovirus and measles virus. AES is reported mainly from Assam, Bihar, Karnataka, Uttar Pradesh and Tamil Nadu which contributes approximately 80% of cases. The case fatality and morbidity is very high.