NEW DELHI — Kasapu Jagadheeshwar Rao gets up early most mornings to visit the nearby temple, but on Sunday, as he began to ritually bathe the statue of the deity, he felt his vision get blurry and he collapsed on the floor. He has no recollection of the next three hours, until he woke up in the hospital.

More than 500 people have been hospitalized in southern India this month by a mysterious illness that caused those affected to suddenly lose consciousness, have seizures or vomit. Many reported a temporary loss of memory and muscle pain in the back or neck.

The outbreak occurred in the city of Eluru, in the state of Andhra Pradesh, on the southeastern coast of the country, sparking alarm about some kind of new outbreak at a time when the coronavirus pandemic is still raging. About 75 people remain hospitalized, and one person died during treatment after suffering a fall.

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 in nature,” 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 when a gas leak in the central city of Bhopal killed thousands of people in 1984.

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 grown.

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 are 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 patients 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.

The illness struck Teku Venkateshwar Rao suddenly and without warning. A 38-year-old who ekes out a living by ironing clothes, he suffered an epileptic fit on Saturday morning.

"He screamed loudly," said his wife, Teku Varalakshmi. "I was very scared at that time. Never in his life has he suffered epilepsy or seizures."

Fear and confusion still plague the family of 56-year-old Jagadheeshwar Rao, who had fainted at the temple. Two days after being discharged, Rao, who runs a bicycle repair shop, still felt weak. “Now I feel a little better, but my energy levels have dropped after the sickness,” he said.

While he believes the mystery illness could be related to the coronavirus pandemic, he also isn’t taking any chances and has stopped drinking the water supplied by the municipality.

B. Kartheek in Hyderabad contributed to this report.