A mysterious outbreak of a viral pneumonia that’s been linked to a food market in central China has sickened at least 44, according to health authorities, with at least 11 of the cases considered critical. The Wuhan city health commission said those listed as critical are being kept in isolation, and that an additional 121 people who came into contact with those patients are under observation.

Patients have been reporting symptoms ranging from fever, to shortness of breath and lung infections, but health officials remain baffled as to what’s causing the illness. Many cases have been traced to South China Seafood City food market, which has since been disinfected and is under investigation, according to The Associated Press.

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However, two female patients in Hong Kong who had recently traveled to Wuhan were admitted to the hospital after presenting symptoms of fever and respiratory infection or pneumonia, yet authorities said neither had visited the South China Seafood City market. The World Health Organization is also monitoring the situation.

“Investigations are still being carried out and authorities cannot yet confirm what pathogen is causing this illness,” WHO spokesman Tarik Jasarevic told Reuters.

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Health officials in China said there are no clear indications of human-to-human transmission of the disease, and while the market does offer wild animals, they had ruled out avian flu, the common flu and the adenovirus thus far. Rumors on social media suggested a link to Severe Acute Respiratory Syndrome (SARS), but officials were quick to shut those down and said several were facing punishment for spreading false information.

In 2003, Chinese officials were forced to apologize after it was revealed that they had covered up an outbreak of SARS for weeks. More than 8,000 people were infected and over 770 died after it spread to other cities and countries, according to Reuters.

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Officials in Singapore said they would begin screening passengers arriving on flights from Wuhan for fever. Taiwan has reportedly implemented similar procedures.

The Associated Press and Reuters contributed to this report.