Investigations into the unidentified illness detected in the ancient city of Eluru, Andhra Pradesh, have led officials to believe it could have been caused by heavy metals.
Over 500 people have presented at hospitals in the Southern India state since Saturday (6 December), with the majority of patients recovering.
It is not yet confirmed that heavy metal poisoning is the cause – though high percentages of lead and nickel have been traced in the blood samples of some patients.
This is what we know so far.
Where has the illness come from?
Although the specific source of the infection has not been detected, it has been reported that at least 10 patients with the illness had high levels of metals in their blood.
However, Andhra Pradesh health commissioner, Katamaneni Bhaskar, said that “it is not yet clear” how the heavy metals entered their bloodstream.
He said: "We have tentatively identified the primary cause of the strange phenomenon but it is not yet clear of the source and how and why it has happened.
"For instance, the chemicals are found in the body of the patients but not in either the water or any food substance."
More patients suffering from the mystery illness have had blood samples sent for testing – according to Superintendent of Eluru Government Hospital, Dr A V Mohan.
He also told The Indian Express: "The sample size that we sent on an urgent basis to AIIMS (All India Institute of Medical Sciences in Delhi) was small but their report indicates presence of heavy metals like lead and nickel in the blood samples of patients.”
How many people are affected?
At least one person has died and around 550 others have received treatment since the illness was detected on the evening of Saturday, 5 December.
A 45-year-old man died on Sunday, 6 December, according to the Press Trust of India news agency. No fatalities have been recorded since this one.
Of the people who have presented at hospitals in Southern India, over 300 are thought to be children and only 84 remain in hospital.
What are the symptoms?
The Indian health ministry has stated that “The children reportedly suffered from dizziness, fainting spells, headache and vomiting.”
Other symptoms include anxiety and unconsciousness.
The person who died was thought to have displayed symptoms similar to epilepsy.
Doctors have confirmed that the deceased and sick were tested for coronavirus but proved negative.
Covid-19 has already killed over 140,000 people in India.
The illness is also unrelated to Japanese encephalitis, dengue or chikungunya, which are caused by mosquito bites.
What has the Indian government said?
State chief minister YS Jagan Mohan Reddy visited a government hospital and met patients who were ill on Monday.
Meanwhile, opposition leader N Chandrababu Naidu has called for an “impartial, full-fledged inquiry into the incident”.
India’s federal health ministry has also sent three medical experts to lead investigations, with door-to-door surveys being carried out in the state.
What is the state of healthcare in India?
There is a universal healthcare model in India which is predominantly run at state level, as opposed to federal.
Government hospitals tend to serve less wealthy populations, especially women and the elderly.
The current coronavirus pandemic has had a profound impact on the Indian healthcare system. The country dealt with over 9.6million cases, second only to the US - 800,000 of which were in the Andhra Pradesh region.
The death toll from coronavirus in India is around 140,500, with only the US and Brazil facing higher fatality levels.