The health commission in Wuhan, a transit and business hub, says there is no clear evidence that the unidentified disease can be transmitted between humans, and no health-care workers have been infected. Cases of fever have been reported in Hong Kong and Taiwan by travelers who recently visited Wuhan, although there is no confirmation that the illnesses are linked.

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Since mid-December, 59 people have been diagnosed with viral pneumonia of “unknown cause” — including seven who are critically ill, according to Wuhan’s health commission. The officials said an additional 163 people who have come into close contact with the infected have been placed under close observation. No deaths have been reported.

Several of the patients worked at Wuhan’s South China Seafood City, said the authorities, who shut down the market Jan. 1 to carry out daily disinfections. The 1,000-stall bazaar sold not only seafood but marmots, spotted deer and venomous snakes, according to state media reports that described the market as “filthy and messy.”

Videos from Wuhan showed the market barricaded in recent days and guarded by police wearing surgical masks.

The emergence of a new illness out of China has carried echoes of the severe acute respiratory syndrome (SARS) epidemic, which infected more than 8,000 people, killed 774 and sparked mass panic as it spread across more than two dozen countries over eight months after starting in China in November 2002.

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SARS is thought to be an animal virus from an as-yet-uncertain animal reservoir, perhaps bats, that spread to other animals, such as civet cats, and first infected humans in China’s Guangdong province, according to the World Health Organization. The tropical civet is sold and eaten as a delicacy in south China. The SARS epidemic delivered a political shock to the Communist Party, which was widely condemned for mismanaging the outbreak, covering up cases and smothering news reports.

Wuhan’s health commission, which has given relatively regular news updates, said the new pneumonia cases were not caused by SARS. It has also ruled out influenza, bird flu, adenoviruses and Middle East respiratory syndrome (MERS) and said nucleic acid analysis was underway to identify the pathogen.

“There are now two key questions: What is the cause of the disease? Is it a brand new virus?” said Leo Poon, a Hong Kong University epidemiologist who was among the first to decode the SARS coronavirus. “Next: Is it transmissible between humans? We cannot yet rule out the possibility entirely.”

Because Wuhan authorities began quarantining and disinfecting the market on New Year’s Day, the number of new cases should taper off in the coming weeks, Poon said.

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“But if there are additional cases, that may suggest human-to-human transmission” that would greatly complicate the situation, he added.

The WHO said it is monitoring the situation in China. The U.S. Centers for Disease Control and Prevention issued a low-level travel notice Monday, reminding travelers to practice “usual precautions” inside the country.

On Tuesday, the CDC said it has established an incident management structure to “optimize domestic and international coordination in the event additional public health actions are required,” according to a notice the agency sent to state health officers and public health emergency preparedness directors.

There are no known U.S. cases or any cases in any countries outside China, the CDC said. “But outbreaks of unknown respiratory disease are always of concern, particularly when there are possible zoonotic origins to the outbreak,” the CDC statement said.

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On Wednesday, the CDC issued a health advisory to clinicians to consider pneumonia as a diagnosis for patients with severe respiratory symptoms who have been to Wuhan since Dec. 1, had illness within two weeks of returning and do not have another diagnosis that would explain their illness.

Public health experts say it is reassuring that no health-care workers have fallen sick. When the SARS outbreak began, the major alarm bells were illness in doctors and nurses and the spread of disease from person to person, said Tom Inglesby, director of the Center for Health Security of the Johns Hopkins Bloomberg School of Public Health.

But while Chinese officials have ruled out several causes of the illnesses, they have not provided detailed information about what tests were performed, when they were done and at what point in the patients’ illnesses. Nor have Chinese officials provided a timeline of patient illnesses, information that is typically made public quickly in disease outbreaks.

“We’ve heard they ruled out SARS and MERS and other coronaviruses. There seem to be a lot of reports that this is a novel pathogen, but we haven’t seen the evidence,” said Jennifer Nuzzo, an epidemiologist and senior scholar at the same center.

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In the absence of additional information from Wuhan officials, other countries have announced additional measures, such as quarantine and fever screening at airports. If there is predictable and daily communication from Chinese health leaders in charge of the response about what is known and what is unknown about the outbreak, that “would tamp down some of the anxiety” about the disease and the response, Nuzzo said.

If the Wuhan pneumonia were found to be contagious, it could pose a major public health challenge coming just before the Lunar New Year holiday, when more than 400 million Chinese are expected to travel — including 7 million who vacation overseas.

In Beijing, the capital’s health officials this week called for “public health readiness” to respond to emergencies during the holiday period of Jan. 24-30.

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In Hong Kong, 30 people who have visited Wuhan in recent weeks have been hospitalized with fever, and pharmacies in the city quickly sold out of face masks, local media reported.

The city’s chief executive, Carrie Lam, said Tuesday that authorities have rolled out new measures that would require Hong Kong’s doctors to report suspected cases and give city officials legal powers to quarantine suspected patients.

All travelers by high-speed rail from Wuhan will have their body temperature tested before entering Hong Kong, Lam said.

The Taiwanese government said eight people traveling from Wuhan have exhibited fever, and it was offering to send epidemiologists to Wuhan to help investigate.

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Xu Jianguo, a former top Chinese public health official, struck an assuring note and said the government’s disease control capabilities today are much stronger than they were in the early 2000s.

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“More than a decade has passed,” he said. “It’s impossible for something like SARS to happen again.”