The seasonal flu has killed more people than the coronavirus, but that is not why the outbreak is so concerning, infectious disease expert Ian Lipkin told CNBC on Monday.

"It's a new virus. We don't know much about it, and therefore we're all concerned to make certain it doesn't evolve into something even worse," said Lipkin, speaking from his New York home on a 14-day self-quarantine after traveling to China to work on the outbreak.

Lipkin, the director of the Center for Infection and Immunity at Columbia University's Mailman School of Public Health, was in Guangzhou and Beijing, where he advised local health officials. He said he did not travel to the city where the coronavirus emerged, Wuhan in central China, because it would have been more difficult to return to the U.S.

Lipkin, who worked on the 2003 SARS outbreak, said it is true that seasonal flu presents its own kind of problem, noting that globally up to 650,000 people die from it each year.

So far, more than 900 people who had the coronavirus have died.

The coronavirus is "not nearly as challenging for us as influenza" when seen strictly by the number of deaths, Lipkin said.

But that is not the only lens through which the outbreak should be viewed, he cautioned.

"We don't know much about its transmissibility. We don't necessarily have accurate diagnostic tests. And we don't really know where the outbreak is going to go," Lipkin said on CNBC's "The Exchange."

"The only thing we have at present, absent vaccines or drugs, is containment," he added.

The coronavirus emerged in Wuhan about a month ago and has since spread from about 300 people as of Jan. 21 to more than 40,000. New cases grow by the thousands each day, the vast majority of which are in China.

Coronaviruses typically infect animals, but strains can sometimes evolve and spread to humans. Symptoms in humans can include fever, sore throat and shortness of breath.

The respiratory disease is capable of spreading through human-to-human contact, world health officials say.

The illness also is able to spread before symptoms materialize. Health officials estimate about 20% of patients become severely sick, leading to pneumonia and respiratory failure.

Lipkin said he estimates the mortality rate of the coronavirus will ultimately be less than 1%.

But the figure is "speculative" because more antibody tests need to be conducted "so we can figure out who might have been infected but not manifested signs of disease," he said.

Asked whether he thought the Chinese government was being transparent about the disease, Lipkin said he thought it was doing an adequate job of disseminating the information it has.

"What may be taken as a lack of transparency may in fact be ignorance of what's going on," he said.

Lipkin said he plans to return to China within the next month, while some of his staff plan to go there next week.

"There's an enormous amount to do," he said. "And the suffering is impossible to explain."

— CNBC's Berkeley Lovelace Jr. contributed to this report.