The authorities in Singapore placed a Chinese girl with pneumonia in isolation because she had traveled 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 learned its lesson from SARS, said Leo Poon, a public health expert at the University of Hong Kong. “I have to emphasize this is a new disease, and no one on earth has gone through this before,” he said.

Mr. 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.”

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. Dr. Wang 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 Organization said it was closely monitoring the situation and was prepared to mount a broader response, if needed.