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At least 59 people in China have become ill with a mysterious pneumonia – seven of whom are in a serious condition, . Chinese authorities still don’t know what has caused the outbreak, but have ruled out SARS, MERS and bird flu.

“It seems that a new virus or bacteria might be the cause of the disease,” says at Duke University in Durham, North Carolina. “That is worrying somehow.”

The cases, reported to have occurred in Wuhan City, Hubei Province of China, are currently being investigated by Chinese health authorities. So far, there is no evidence that the infection can spread between people, according to the authorities. But it is too soon to definitively say that the infection won’t spread this way, says Tang.

No deaths have been recorded so far, but that doesn’t mean the infection isn’t dangerous, says at the London School of Hygiene and Tropical Medicine. “It’s hard to know how severe a disease is, especially a new infection, because you only see the cases that are severe enough to be detected,” she says. “So we can’t say yet what the fatality rate is.”

Several of those affected worked in a local market known for seafood. The market was closed on 1 January, according to . The Wuhan Municipal Health Committee says it will “carry out environmental sanitation and further hygiene investigations”.

All of the affected individuals became ill between 12 and 29 December. The symptoms include fever, and some of those affected have difficulty breathing. Currently, all are receiving medical treatment in isolation, and the people they have had contact with are being evaluated, according to a report by the Wuhan Municipal Health Committee.

That committee is also investigating the cause of the infections. As of 5 January, it had ruled out influenza, bird flu, adenovirus (which can cause colds, pneumonia and conjunctivitis), MERS and SARS, which was responsible for 774 deaths in 2003.

“There have been no hospital outbreaks, so it looks better than SARS,” says at Hong Kong University. But that could change, he warns. “Viruses and bacteria can adapt and mutate – we have to be very cautious of that.”

The Hong Kong Hospital Authority responded on 4 January by activating a “serious response level” in public hospitals. Hospital staff should note any symptoms of fever or pneumonia, and should check if people with such symptoms have travelled to Wuhan. Those who are suspected of having the disease will be isolated, with limited access to visitors, according to a .

Hong Kong’s Centre for Health Protection is , which will be used to check if people travelling from Wuhan have a fever. Rail travellers may also have their temperature taken by staff. Additionally, the centre is advising that citizens travelling outside of Hong Kong avoid live animal and “wet” markets, and to wear a surgical mask if .

The World Health Organization is “closely monitoring” the event, says spokesperson Fadéla Chaib. Currently, the organisation is advising against imposing any trade or travel restrictions, but echoes the advice that travellers to and from the area should seek medical advice if they develop any symptoms.