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Anecdotal reports from China, and one published study from Germany, indicate that some people infected with the Wuhan coronavirus can pass it on before they show symptoms. That may make border-screening much harder, scientists said.

Epidemiological modeling released Friday by the European Center for Disease Prevention and Control estimated that 75 percent of infected people reaching Europe from China would still be in the incubation periods upon arrival, and therefore not detected by airport screening, which looks for fevers, coughs and breathing difficulties.

But if thermal cameras miss victims who are beyond incubation and actively infecting others, the real number of missed carriers may be higher than 75 percent.

Still, asymptomatic carriers “are not normally major drivers of epidemics,” Dr. Fauci said. Most people get ill from someone they know to be sick — a family member, a co-worker or a patient, for example.

The virus’s most vulnerable target is Africa, many experts said. More than 1 million expatriate Chinese work there, mostly on mining, drilling or engineering projects. Also, many Africans work and study in China and other countries where the virus has been found.

If anyone on the continent has the virus now, “I’m not sure the diagnostic systems are in place to detect it,” said Dr. Daniel Bausch, head of scientific programs for the American Society of Tropical Medicine and Hygiene, who is consulting with the W.H.O. on the outbreak.

South Africa and Senegal could probably diagnose it, he said. Nigeria and some other countries have asked the W.H.O. for the genetic materials and training they need to perform diagnostic tests, but that will take time.