What baffle experts is that there doesn’t seem to be any common link among the hundreds of people who have fallen sick. All of the patients have tested negative for COVID-19 and other viral diseases such as dengue, chikungunya or herpes, they aren’t related to each other. They don’t all live in the same area. They’re from different age groups, including about 70 children, but very few are elderly. Initially, contaminated water was suspected. But the chief minister’s office confirmed that people who don’t use the municipal water supply have also fallen ill, and that initial tests of water samples didn’t reveal any harmful chemicals.

A 45-year-old man with the single name Sridhar was hospitalized with symptoms resembling epilepsy and died Sunday evening, doctors said. Prasadini said his autopsy didn’t shed any light on the cause of death. Now, preliminary medical reports offer a clue —and it has nothing to do with the coronavirus. “What has been established by experts is that this is a case of acute intoxication of toxins. It is not chronic,” said Himanshu Shukla, a high-ranking district official. “This is all we know for now.”

Perplexed by the peculiar nature of the illness and the high number of patients, local health officials are racing to test water, milk, vegetables and fish samples to identify the possible source. Teams from the World Health Organization, molecular biologists, virologists and nutritionists are also part of the investigations. The toxins found in blood samples included lead and nickel, but it is not clear how they got there. Organochlorine, which is normally found in pesticides, was also found in some of the water samples that were tested.

Industries are a major source of air and water pollutants in India, and accidents are not uncommon. In May, a toxic gas leak from a chemical plant killed 11 people and sickened hundreds in the port city of Visakhapatnam, in the same state. India was also home to the world’s worst industrial disaster in 1984 when a gas leak in the central city of Bhopal killed thousands of people.

When the first patients began to arrive at the Eluru government hospital on Saturday evening, many were in a panic, recalled A.V.R. Mohan, the medical superintendent. By the next morning, the numbers had only swelled. Mohan immediately roped in nearby private hospitals to accommodate those streaming in. “Most patients recovered within a day,” he said. “They were given anti-epileptic and antiemetic treatment.” Several patients also had psychological trauma, he added. Things were now under control, said Shukla, the district official. “This is a point source epidemic,” he said. “Whatever happened, occurred for one particular day and some people got affected. The number of new patients has dropped.”

On Wednesday, just 16 new people were admitted to the hospital with the telltale symptoms, a drop from the previous days. To monitor those who have recovered and detect new patients, the district administration has set up about 80 medical camps in the area. A few patients reported repeat seizures after discharge, Mohan said.

Opposition leader N. Chandrababu Naidu demanded on Twitter an “impartial, full-fledged inquiry into the incident.” Andhra Pradesh state is among that worst hit by COVID-19, with over 800,000 detected cases. The health system in the state, like the rest of India, has been frayed by the virus.

By Jumana Jabeer