NEW DELHI — Three years ago, Dr. Rajesh Yadav, an investigator with the India Epidemic Intelligence Service, moved to the city of Muzaffarpur, the site of one of the country’s most mysterious outbreaks. And he waited.

Every year in mid-May, as temperatures reached scorching heights, parents took children who had been healthy the night before to the hospital. The children awakened with a high-pitch cry in the early morning, many parents said.

Then the youths began having seizures and slipping into comas. In about 40 percent of cases, they died.

Every year in July, with the arrival of monsoon rains, the outbreak ended as suddenly as it began.

Beginning in 1995, investigations variously ascribed the phenomenon to heat stroke; to infections carried by rats, bats or sand flies; or to pesticides used in the region’s ubiquitous lychee orchards. But there were few signposts for investigators.