Fox News medical analyst Dr. Marc Siegel joined "Tucker Carlson Tonight" Thursday to give an update on the coronavirus that originated in the Chinese city of Wuhan and has spread to several other countries, including the United States.

"If we were in China right now, I think we should really be worried because the Chinese government is locking down five cities," said Siegel who explained that the virus "began with exotic animals in a market."

"And these animals, by the way, harbor this virus in the past from one species to another, and then humans get infected occasionally."

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"Here is the problem: It's now spreading from human to human," Siegel added. "And we don't know how many cases there are."

The total number of coronavirus deaths stand at 25. Meanwhile, at least 830 cases have been reported worldwide. At least one person has been infected in the United States.

Authorities in the central Chinese province of Hubei have imposed travel bans in eight cities, including Wuhan, the South China Morning Post reported. San Francisco International Airport, John F. Kennedy International Airport in New York and Los Angeles International Airport, began screening for the coronavirus on Jan. 17.

Siegel said he was concerned that the number of people infected in China is much higher than publicly reported.

"I frankly don't believe that it's only 600 cases [in China]. I'm suspicious that it's many more cases than that and many more deaths than that," Siegel said. "So that's one reason that I would really urge people not to travel to China right now until we figure this out."

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The doctor also said he was less concerned about the virus in the United States.

"Here's the billion-dollar question, Tucker. Exactly how contagious is this from human to human?" Siegel said. "Now, over here in the United States, I'm much less worried because as you said, we have one, possibly two cases."

"We're concerned about it, but we still don't know how contagious it is. And public health measures help to contain that," Siegel added. "And right now, I'm optimistic that we can contain this in the United States. We still don't know, though."

Fox News' Bradford Betz and Madeleine Farber contributed to this report.