BEIJING — The Chinese authorities said on Sunday that 17 more people had been infected with a mysterious new virus, raising questions about how it is being transmitted and adding to concerns about the spread of the illness ahead of China’s busiest travel season.
The announcement, by the health commission in Wuhan, a central Chinese city where the virus originated last month, comes amid growing worry among some experts that the outbreak of the illness, the pneumonialike coronavirus, could be more severe than China’s government has described.
The virus has killed two people and sickened at least 62 in the country, according to official statistics. And with hundreds of millions of people in China expected to travel for the Lunar New Year holiday, which begins Friday, public health officials are working to stop a major outbreak.
The virus already appears to have spread outside China. Officials in Thailand and Japan have confirmed three cases involving people who have traveled through Wuhan.
The health commission in Wuhan said in a statement on Sunday that the 17 infected people had begun showing symptoms of the coronavirus as recently as last week. Three are in critical condition, the commission said.
Most people with the infection have contracted it through exposure to animals at a market in Wuhan that sells seafood and live animals, the authorities say.
Health experts are studying whether the virus can spread from human to human on a broad scale.
The World Health Organization said on Sunday that while its analysis indicated that limited transmission of the virus is possible, it remains unclear whether it can easily spread from one person to another. The group said it would continue to examine the issue.
“We do not have enough evidence to evaluate the full extent of human-to-human transmission,” its Manila office said on Twitter.
The health commission in Wuhan said on Sunday that some people who had come down with the virus had no exposure to the market.
That acknowledgment raises the possibility that the virus could be present in other markets in Wuhan, experts said, adding to fears that more people might be at risk.
“If you cannot find the source and control the source of the virus, you cannot extinguish the fire,” said David Hui, the director of the Stanley Ho Center for Emerging Infectious Diseases at the Chinese University of Hong Kong.
Dr. Hui said the risk of widespread human-to-human transmission appeared to be low, though he noted that the virus could mutate.
Some experts have suggested that there are probably far more cases of the illness than the authorities have disclosed. In previous incidents, like the 2003 outbreak of severe acute respiratory syndrome, or SARS, the Chinese government withheld critical information.
One estimate by researchers at Imperial College London suggested on Friday that there could now be as many as 1,700 cases of the new virus.
The W.H.O. said on Sunday that China could potentially confirm more cases in the coming days and weeks as more people were screened for it.
The new virus has brought back memories of the SARS outbreak, which was also caused by a coronavirus. SARS, which is believed to have jumped to humans from animals at markets, originated in China and spread to other countries, infecting more than 8,000 people and killing more than 800.
While the new coronavirus appears to be less severe than SARS, public health officials around the world are exercising caution.
On Friday, the Centers for Disease Control and Prevention in the United States announced that airports in New York, San Francisco and Los Angeles would begin screening passengers from Wuhan for the virus.
W. Ian Lipkin, a Columbia University professor who assisted in the SARS outbreak, said it was too early to know how deadly the new virus might prove to be.
“Until it becomes capable of human-to-human transmission, there’s not a major threat of a pandemic,” said Dr. Lipkin, the director of the Center for Infection and Immunity at the university’s Mailman School of Public Health.
“We need to prepare for the possibility that this could be a larger outbreak, and it could become a pandemic,” he said. “But that doesn’t mean that it will.”
Elsie Chen contributed research.