HONG KONG — When the man finally went to a hospital, he had been sick for a week. It was Dec. 26, and Zeng, 61, was weak with a cough. He got worse. A day later he was transferred to intensive care, and on Dec. 30 he was put on a ventilator to try to keep him alive.

He was moved to another hospital and attached to another machine that oxygenated his blood. Still, he got worse, and on Jan. 9 his heart stopped.

Zeng, whom authorities have identified only by his surname, became the first confirmed death from the new coronavirus that emerged in the central city of Wuhan and has since spread around the country and beyond.

China’s health commission, which has tightly controlled news about the toll of the outbreak, on Thursday released details about the first 17 confirmed deaths from the disease. (More deaths were announced early Friday, bringing the death toll to 26.)

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The detailed information was released as the authorities canceled transportation within Wuhan and several nearby cities and largely blocked residents from leaving. Medical experts have questioned whether the measures in Wuhan have come too late to prevent the spread of the coronavirus, which has been found in infected travelers in Washington state, Japan, South Korea, Thailand and Taiwan.

Dr. Guan Yi, a professor of infectious diseases at the University of Hong Kong who visited Wuhan this week, warned that there was a potential for the virus to spread rapidly despite the controls put in place on Thursday morning.

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“We have a chance to have a pandemic outbreak,” said Guan, who was part of the team that identified the coronavirus that caused the deadly SARS outbreak in 2002 and 2003. SARS infected more than 8,000 people and killed nearly 800.

Guan also told Caixin, an influential Chinese magazine known for investigative reports, that he had traveled to Wuhan hoping to help track the virus’s animal source and control the epidemic. But he left, he said, feeling “powerless, very angry.”

Dr. W. Ian Lipkin, an epidemiologist at Columbia University who advised the Chinese government and the World Health Organization during the SARS outbreak, said that infected people outside Wuhan would continue to spread the disease.

“The horse is already out of the barn,” he said.

An examination of the information provided by the government about the initial deaths show a disease that has thus far largely killed older men, many of who had underlying health problems.

Most had gone to the hospital with a fever and a cough, although at least three did not have fevers when they were admitted, according to the health commission’s statement.

Among the first 17 victims were 13 men and four women. All were identified only by their last names. The youngest was a 48-year-old woman, Yin, who died on Monday, more than a month after her symptoms were first recorded. The oldest cases were two 89-year-old men who died Saturday and Sunday. The median age was 75.

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Many had underlying conditions like cirrhosis of the liver, hypertension, diabetes and Parkinson’s disease. Most spent more than a week in hospitals, with some undergoing treatment for a month or longer. But two died just four days after they were admitted.

While much about the virus remains unknown, medical experts found some positive signs in the fact that the disease did not appear to be killing young and otherwise healthy people.

It was a somewhat reassuring sign, Lipkin wrote, that “the majority of fatal cases are elderly and/or have a chronic disease that would increase their susceptibility to infectious diseases.”

The Chinese health commission said more than 570 cases had been confirmed in the country by the end of Wednesday, with 95 in grave condition. The outbreak has happened as China was preparing for the Lunar New Year holiday, the biggest travel period of the year, increasing the likelihood of the coronavirus circulating further beyond Wuhan.

Guan, in his interview with Caixin, was critical of the local government, saying it had not done enough earlier this week to stop the coronavirus in Wuhan.

“Even though the central authorities have said in the past two days they were attaching a high degree of importance, local health protections had not been upgraded at all,” he said. “At the time I thought this was going to be a ‘state of war.’ Why hadn’t the alarm been sounded?”

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Guan said he was disturbed by the lack of safety measures being put in place. At the airport he saw no disinfection being carried out and only a few random places like a Starbucks had put out liquid hand sanitizer dispensers.

The situation was so surprising, “my jaw dropped,” he said.

He said he continually ran into obstacles when trying to find researchers to work with on tracing the source of the virus. The seafood and poultry market believed to the source had been thoroughly cleaned, he complained, preventing any effective investigation.

“There’s no crime scene,” he said.

The path of the coronavirus could prove harder to trace and control than SARS, when a small number of highly infectious superspreaders helped transmit the disease to a large number of people, Guan said.

“I’ve experienced a lot, and I’ve never felt scared, most of these are controllable,” he said, citing previous battles with SARS, avian influenza and other outbreaks. “But this time I’m scared.”