In June 2019, an outbreak of acute encephalitis syndrome (AES) occurred in Muzaffarpur and the adjoining districts in Bihar state of India resulting in deaths of more than 150 children, mainly due to hypoglycemia. In subsequent months more cases and deaths were reported. The cause of outbreak is unclear. Malnutrition, climate, hygiene, inadequate health facilities, and lack of awareness are considered as contributing factors. The lychee fruit toxins are also cited as plausible cause of AES. History Outbreaks of acute encephalitis syndrome (AES) have occurred previously in the northern regions of Bihar state and eastern regions of Uttar Pradesh state of India. The first case of AES in Muzaffarpur district was recorded in 1995. There were 143 deaths in 2013, 355 in 2014, 11 in 2015, four in 2016, 11 in 2017 and 7 in 2018. In most recent years, the death toll had remained under 20. Outbreak In June 2019, an outbreak of AES occurred in 222 blocks of Muzaffarpur and the adjoining districts in Bihar. As a result of the outbreak, total 154 children died in first three weeks of June 2019. A total of 440 cases of AES were admitted to hospitals in these three weeks. At least 85 children of them died at the Sri Krishna Medical College and Hospital (SKMCH), the largest state-operated hospital in Bihar, while at least 18 children died at the Kejriwal Matrisadan, a trust-run hospital, in these weeks. Most of them were aged between 1 and 10 years. In subsequent months of July, August and September; at SKMCH; 30, 18 and at least 12 cases were reported. Total 647 cases of AES including 161 deaths were reported between 1 June and 20 September 2019. Syndrome The term acute encephalitis syndrome (AES) was coined by the World Health Organisation in 2008. The symptoms of AES include an acute onset of fever and associated clinical neurological manifestations such as mental confusion, disorientation, delirium, convulsions, or coma. Early symptoms include headaches and vomiting, along with sudden hypoglycemia (drops in blood sugar levels), but may lead to coma, brain dysfunctions, and inflammation of the heart and lungs. Those that survive AES may have long-term neurological weaknesses. The severe hypoglycemia can cause death. The syndrome is locally known as Chamki Bukhar in Bihar. Cause The cause of the outbreak is unclear. AES can be caused by different microorganisms including virus, bacteria, fungi, parasites and spirochetes, as