World Health Organization experts are surveying neighbourhoods in and around Eluru
One person has died and nearly 600 have been hospitalised since Saturday in Andhra Pradesh due to a mysterious infection that caused many to fall unconscious after suffering giddiness, seizures and nausea.
What exactly happened?
The first patients were admitted to hospital on Saturday in Eluru, a town in Andhra Pradesh. By Sunday, according to the health ministry, there were more than 200 people hospitalised, suffering dizziness, headache and vomiting.
Patients including children and women fainted, and that evening a 45-year-old man died. By Wednesday, the number of patients had jumped nearly three-fold. As of Thursday, over 85% of the patients had been discharged.
The cause of their illness has yet to be identified. Tests ruled out COVID-19 infection.
What are the potential causes?
Eluru is home to over 200,000 people, and is known for its weaving industry. Investigations have focused on whether the illness could be linked to some groundwater contamination, excessive use of pesticides or mass application of chlorine as part of a coronavirus disinfection drive.
Doctors have also found traces of lead and nickel in the blood samples of some patients. The elevated lead levels could have been a trigger, especially as the levels appeared to drop among patients a day after hospitalisation, doctors say. The state government says officials are also checking if dumping or burning of used batteries could have caused the illness, without elaborating further. They are also testing the area's drinking water and milk samples.
A senior health official has also said exposure to organochlorides found in pesticides and used in mosquito control could be a potential cause.
What are the effects of exposure to lead or chlorine?
High levels of lead in the bloodstream can impair development of brains, nervous systems and vital organs such as heart and lungs. Organochlorides are banned or restricted in many countries after research linked them to cancer and other health risks.
World Health Organization experts are surveying neighbourhoods in and around Eluru, collecting various information, including on food sources.
Health authorities are running various tests and the state's chief minister has promised corrective measures once investigations are completed.