The deaths have occurred in and around the city of Muzaffarpur in the northern state of Bihar, one of the poorest regions in India. The illness strikes the area each summer, but in recent years, the death toll has been less than 20.

This year, at least 97 children have died, most of them under the age of 7, said Alok Ranjan Ghosh, a senior government official in Muzaffarpur. About 100 more remain in hospitals. The deaths represent the worst toll since 2014, according to figures provided by a state health official.

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Doctors describe the illness as acute encephalitis syndrome, or AES, which is characterized by the rapid onset of fever combined with disorientation and seizures.

The syndrome has myriad causes, including viruses spread by mosquitoes and human contact, bacterial infections, parasites and toxins.

In the Muzaffarpur case, officials say the precise cause of the outbreak remains unclear. Elsewhere in India, a mosquito-borne virus called Japanese encephalitis sometimes played a leading role in outbreaks of AES. But in Muzaffarpur, the virus appears absent in the vast majority of the cases, said Sanjay Kumar, principal secretary at the Bihar state ministry of health.

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The illness has been observed in the area since 1995, Kumar said, flaring up during the Indian summer when temperatures soar and subsiding with the arrival of the rains and somewhat cooler weather. Children are arriving at hospitals with critically low blood-sugar levels, he said.

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Sunil Kumar Shahi, the superintendent of Sri Krishna Medical College and Hospital in Muzaffarpur, where most of the children are being treated, said that all of the patients with the illness were under the age of 9.

“You can imagine my mental condition. The situation is horrible,” he said, before rushing off to treat more children. “Think of the parents whose kids are admitted.”

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Gopal Shankar Sahni, the head of the pediatrics department at the same hospital, said its halls were crowded with distressed and crying parents. “They ask us to treat their kids, but we ask them to believe in God,” he said.

A recent study conducted by India’s National Center for Disease Control in conjunction with the Centers for Disease Control and Prevention in the United States pointed to a different possible cause for the illness. The area around Muzaffarpur is a major litchi-growing region, and the study found that toxins occurring naturally in the fruit were a plausible source of the illness, especially when children failed to eat an evening meal the day before becoming sick.

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The outbreaks of this “acute unexplained neurological illness . . . begin in mid-May and peak in June, coinciding with the month-long litchi harvesting season,” the researchers noted, adding that most of the affected children come from poor and rural families.

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They posited that the illness might only occur with a combination of factors, for instance when a poorly nourished child eats a large number of litchis and then misses an evening meal. They also pointed to the limitations on local hospitals in providing critical care to such patients, which likely contributes to the death toll.

Indian television channels showed scenes of unresponsive children lying several to a bed at a Muzaffarpur hospital. When Indian Health Minister Harsh Vardhan visited the area on Sunday, he was trailed by angry and grieving relatives.

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Vardhan vowed that the national government would provide “all possible help” to local authorities, according to media reports. Earlier, he had expressed confidence that the government would “soon be able to contain the rise” in AES cases.

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In August 2017, India witnessed a notorious outbreak of encephalitis in the city of Gorakhpur in the neighboring state of Uttar Pradesh. More than 30 children died over two days at one hospital after its oxygen ran out.