Many questions about the new virus remain. While it appears to be transmitted to humans via animals, the Chinese government has not said which animals, nor has it disclosed other details about the outbreak, like the transmission route, the incubation period or the ages and genders of the patients.

“So, there are still a lot of question marks,” said David Hui, an expert in emerging infections at the Chinese University of Hong Kong.

The authorities in Wuhan are still closely monitoring 163 people who were in close contact with the patients, Dr. Hui said. He added that 15 days, the minimum incubation period for some viral infections, had not yet passed since the last reported instance of the disease, on Dec. 29.

“It’s premature to say that there’s no human-to-human transmission,” Dr. Hui said.

The Wuhan government confirmed on Dec. 31 that the health authorities were treating dozens of cases of pneumonia of unknown cause. Symptoms of the new illness include high fever, difficulty breathing and lung lesions, according to the Wuhan health commission. Seven people have become critically ill, the Wuhan authorities have said. On Wednesday, the local health commission said eight people had been discharged.

Researchers have been encouraged by the fact that patients’ relatives and hospital workers have not been reported to have gotten sick, signaling that the virus may not spread easily among humans.

“We can assume that this virus transmissibility is not that high,” said Guan Yi, a professor of infectious diseases at the University of Hong Kong, who was part of a team that successfully identified the coronavirus that caused SARS.

Dr. Guan said he was not surprised that the virus identified was a coronavirus, because coronaviruses can pass from animals to humans easily. But he said it would probably be a while before researchers came up with treatments for the illness.