Indian authorities are investigating if organochlorines used as pesticides or in mosquito control caused the death of one person and hospitalisation of more than 400 in the southern state of Andhra Pradesh in the past few days, a health official said.
The unknown illness has infected more than 300 children, with most of them suffering from dizziness, fainting spells, headache and vomiting.
They have tested negative for Covid-19.
The government has sent doctors from the National Institute of Virology, National Centre for Disease Control and the All India Institute of Medical Sciences have been sent to investigate.
The team was collecting samples of edible oil, rice, blood and urine for analysis.
According to district officials, the illness is not spread person-to-person.
A public health official said experts were awaiting test reports to ascertain the cause, but organochlorines is being considered as one possible reason behind the illness.
No new serious cases have come to light in the past 24 hours.
A 45-year-old man died over the weekend.
Organochlorines are banned or restricted in many countries after research linked them to cancer and other potential health risks.
However, some of the pollutants remain in the environment for years and build up in animal and human body fat.
It was not immediately clear how extensively the chemicals are used in India, though it is found in DDT applied for mosquito control.
Exposure to organochlorine pesticides over a short period may produce convulsions, headache, dizziness, nausea, vomiting, tremors, confusion, muscle weakness, slurred speech, salivation and sweating, US health authorities say.