MUZAFFARPUR, India — The children begin arriving every year in mid-May, brought to an overburdened hospital here in one of India’s most impoverished areas by their panic-stricken mothers. Seemingly healthy hours earlier, most have lapsed into a coma, punctuated by convulsions.

Doctors work to calm the convulsions and keep the children hydrated, but then have to watch helplessly along with the anguished parents as a third of their young patients die, often within hours. Then, as suddenly as it started, the mysterious outbreak stops with the onset of the monsoon rains this month. No one knows why.

The mud-and-bamboo huts in this part of India cradle more dying children than anywhere in the world, with diarrhea and malnutrition long ranking as the region’s great scourges. But now, this unknown killer is stalking the very young.

Although public health statistics are unreliable here, the disease is believed to infect tens of thousands of people a year and kill thousands.