BEIJING (NYTIMES) - For days, Li Bin had what felt like a cold, with a high fever of between 38.8 deg C and 40 deg C, and he could not understand why he wasn't getting better.

After four days, he went to a hospital, and a doctor told him he had a form of viral pneumonia, without offering specifics. Mr Li, 42, was hospitalised, then transferred to another facility and quarantined with other patients who had similarly unexplained symptoms.

He is one of 59 people in the central Chinese city of Wuhan who have been sickened by a pneumonia-like illness, the cause of which is unclear.

The cases have alarmed Chinese officials, who are racing to unravel the mystery behind them in a region where the memory of an outbreak of the dangerous respiratory disease known as Sars remains fresh.

Sars, or severe acute respiratory syndrome, originated in China and killed more than 800 people worldwide in 2002 and 2003. At the time, the Chinese government tried to cover up the problem, which is exacerbating fear now about this new illness.

Symptoms of the new illness include high fever, difficulty breathing and lung lesions, the Wuhan health commission has said. No deaths have been reported but seven people are critically ill.

On Sunday (Jan 5), the city government said they had ruled out as causes Sars, Middle East respiratory syndrome (Mers), bird flu and the adenovirus.

The illness appeared just weeks before the Spring Festival, the country's biggest holiday, when tens of millions of people travel. The authorities urged the public to be on alert for pneumonia-like symptoms like fever, body aches and breathing difficulties.

Workers wearing hazmat suits disinfected and shut down the Huanan Seafood Market in Wuhan, which also sold poultry, pheasants and wild animal meats, after the city health department said it traced many of the cases to it.

Viruses that caused Sars and the H7N9 strain of bird flu in humans were first detected in markets that sold animals and experts have said contact with infected animals was the likeliest source of transmission.

Health authorities around the region have responded quickly. In Hong Kong, 21 people who had visited Wuhan in recent weeks were hospitalised between Dec 31 and noon on Monday, according to that city's health department.

Officials said they would install additional thermal imaging systems at the international airport to monitor passengers coming from Wuhan; they also added staff at the high speed rail station to check body temperatures.

The authorities in Singapore placed a Chinese girl with pneumonia in isolation because she had travelled to Wuhan, then said on Sunday that doctors had found that the child had a common childhood viral illness.

Chinese health officials at first appeared to be closely guarding information about the illness. The Wuhan government confirmed on Dec 31 that health authorities were treating dozens of cases of pneumonia of unknown cause only after an emergency notice to city hospitals was shared on social media sites a day earlier, apparently triggering some public panic.

But more recently, the government appeared to be moving more quickly to disclose information about new cases, in a sign that it has learnt its lesson from Sars, said Professor Leo Poon, a public health expert at the University of Hong Kong.

"I have to emphasise this is a new disease, and no one on earth has gone through this before," he said.

Prof Poon said a surge in cases in the coming week would suggest either that the source of the virus had not been eradicated despite the shutdown of the market or that the illness could be transmitted between humans.

"I hope this pathogen is a less harmful one so it would not cause a major epidemic similar to Sars," he said. "It would be a nightmare for all of us."

Dr Wang Linfa, an expert on emerging infectious diseases at the Duke-NUS Medical School in Singapore, said he was frustrated that scientists in China were not allowed to speak to him about the outbreak.

He said, however, that he thought the virus was likely not spreading from humans to humans because health workers had not contracted the disease. "We should not go into panic mode," he said.

The World Health Organisation said it was closely monitoring the situation and was prepared to mount a broader response, if needed.

In Hong Kong, residents raced to stockpile surgical masks. On Monday, employees at several pharmacies and convenience stores said they had sold out by lunch time. Watsons, a major chain owned by the family of Hong Kong tycoon Li Ka Shing, said sales for masks and hand sanitisers increased significantly this week compared with last week.

The Hong Kong government on Saturday also introduced a new response system for "novel infectious diseases", citing a desire to avoid repeats of the Sars epidemic or the 2009 swine flu outbreak.

In response to the Wuhan cases, the government declared a "serious" response level, the middle tier of the new system, which officials said signalled a "moderate" risk to the local population.

In Wuhan, Mr Li, who works for a hotel restaurant, was one of the first patients to fall ill. He came down with a fever on Dec 23, after visiting the Huanan Seafood Market.

"It felt like a common cold," he said by telephone. He has since recovered and is set to go home in the next two days. None of his other family members have become sick, according to his wife.

The authorities have acted swiftly to clamp down on discussion about the outbreak. Censors blocked the hashtag #WuhanSARS. The police said they were investigating eight people in Wuhan for "spreading rumours" online about the disease, an announcement that was met with anger on Sina Weibo, one of China's most popular social media sites.

"I don't have the right to speak and I don't even know the truth," one user wrote. "Don't I have the right to panic and save myself?"