Although several countries, including the United States, have begun screening air travelers arriving from Wuhan, Dr. Tedros said that the W.H.O. prefers exit screening for people leaving affected regions.

“There is still a lot we don’t know,” Dr. Tedros said. “We don’t know the source, we don’t know how easily it spreads, we don’t understand its clinical features or severity.”

The new infection is caused by a coronavirus, from the same family that caused epidemics of SARS and MERS, which have killed hundreds of people in dozens of countries.

The first cases of the Wuhan illness occurred in people who had been to a market that sold fish and a variety of live animals for consumption, and researchers think the virus came from one of those animals. But which animal is not known. The market was shut down, and it is not clear whether any of the animals or products there were kept for testing.

Finding the source is important, so sales of that animal and its meat can be halted. China has many such markets, and they were also the origin of SARS.

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So far, Dr. Tedros said, about a quarter of the people infected had become severely ill, but most others had milder symptoms. Most who died had underlying health problems, and many were older than 60.

About 830 people have been infected with the virus in China, and at least 25 have died, according to Chinese authorities.