A mysterious illness that struck southeastern India has left more than 300 people hospitalized and doctors baffled.

Local health officials investigating the illness said that at least one person has died, while the rest are all in "stable" condition.

People afflicted with the unidentified illness were from the city of Eluru in the state of Andhra Pradesh. Health officials said that some of the symptoms that were being experienced by patients included nausea, loss of consciousness and seizures.

The state of Andhra Pradesh is one of the most affected regions of the coronavirus pandemic in India, with more than 800,000 confirmed cases. Officials investigating the latest illness did confirm that none of the patients had tested positive for the coronavirus.

The district surveillance officer of Eluru's West Godavari District, Dolla Joshi Roy, said that about 180 of the patients have been discharged and the rest are in stable condition. The one person that died reportedly experienced fatal cardiac arrest, which doctors say may have been unrelated to the mysterious illness.

 Andhra Pradesh's Health Department said in a press statement that they did not find signs of a viral infection in any of the initial blood samples taken from the patients. The agency added that they have already ruled out infections such as dengue or chikungunya.

Health authorities are reportedly still trying to track down the source of the illness and will be testing water samples in the area for contamination. All of the patients reportedly received water from the same source. The health agency has reportedly collected water and other samples from 57,863 households and has sent them to labs for testing.

"The cause is still unknown but still we are doing all kinds of testing, including testing food and milk," said Roy.

Experts from the All India Institute of Medical Sciences recently arrived in Eluru to conduct further tests on the patients. The team included a neurologist from out of state, who was brought in to conduct neurotoxin tests. The Chief Minister of Andhra Pradesh, Y.S. Jaganmohan Reddy, also arrived in the city to take part in the investigation.