In Anyang, China, five members of a family came down with the coronavirus after hosting a guest from Wuhan in early January.
But the visitor, a 20-year-old woman, never got sick herself.
Some individuals who are infected with the coronavirus can spread it even though they have no symptoms, studies have shown.
Asymptomatic carriers are a well-known phenomenon. But the coronavirus is a new pathogen, and these cases may complicate scientific efforts to detect cases and to curb transmission.
"I don't think there's any question that someone who is without symptoms and carrying the virus can transmit the virus to somebody else," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.
"The question is, how prevalent is that phenomenon? Is that becoming an important driver of the outbreaks, or is it an unusual occurrence?"
When asymptomatic carriers are important factors in an outbreak, he said, "you are going to put greater emphasis and burden on testing people."
'Could be missing a lot of cases'
At the moment, the Centers for Disease Control and Prevention allows for testing only symptomatic people who traveled to China recently or those who have had contact with someone who tested positive for coronavirus. (Officials have said the criteria may be reevaluated.)
"We could be missing a great number of cases that don't fit into those criteria," said Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
"I suspect there are a number of additional cases in this country that are transmitting this virus, just like we're seeing in other countries. Absence of evidence is not evidence of absence."
Why are asymptomatic carriers so dangerous
People who are infected but asymptomatic can spread disease efficiently. They are hardy and mobile. They have no reason to avoid crowds or kissing. They don't know they are sick, and no one else does.
These individuals are also hard to detect, suggesting that the current policies to try to contain the spread of the virus may not be adequate. Simply screening international travelers with symptoms of illness - and explicitly precluding tests of patients without a known link to China - may mean new cases are missed.
In February, Germany flew 126 people home from the Wuhan area. Ten passengers were segregated from the others because they didn't feel well or thought they had been exposed to the coronavirus. But everyone was offered testing.
The 10 isolated patients tested negative, but two people - who felt fine - surprised scientists by testing positive. They were hospitalized, monitored and tested repeatedly.
While one developed a mild rash and slightly sore throat, neither became ill.
There have been 59 confirmed coronavirus cases so far in the United States, but little testing has occurred for a country of this size. The CDC has run only 445 tests, not counting tests on people who were repatriated.
Most of the confirmed cases are passengers repatriated from the Diamond Princess cruise ship. The CDC reported Wednesday that two more passengers under quarantine have become ill.
Federal health officials warned Tuesday that hospitals, schools and businesses needed to start preparing for outbreaks in the United States. Containment strategies may have to expand to include steps like closing schools, ordering people to work from home and restricting public gatherings.
The secretary of health and human services, Alex M. Azar II, said he was alarmed by the infections occurring in some parts of the world that have no clear link to confirmed cases.
Until now, the vast majority of infections and deaths have been in China, where the coronavirus originated in Wuhan before spreading to about 40 other nations.
So far, at least 81,109 people have been infected, and at least 2,718 have died.
But other countries may not have confirmed cases because they haven't tested very many people or don't have the resources to run tests.
Some public health experts fear stealth transmissions may already be occurring in communities in the United States. But if sick individuals have no direct link to China, they will not be eligible for testing, so they will not be detected. That may help spread the disease.
"To our knowledge there is no sustained transmission in this country at this point unless it is under the radar," Fauci said.
Aggressive testing can find asymptomatic cases
In Italy, health officials in some regions have taken a different approach.
After 10 deaths attributed to the new coronavirus, health officials started aggressive and widespread testing in some regions. They turned up hundreds of other infections, including many in people who did not display any symptoms.
Quarantines have been imposed on at least 10 towns, and the movement of tens of thousands of people has been limited. There have been no deaths attributed to the coronavirus in the United States.
Earlier reports about asymptomatic transmission - including a published report about a Chinese woman who visited Germany for a few days in January, infecting several colleagues there and not realizing she was ill until she returned home - have been criticized.
A follow-up report said the woman had vague symptoms, like fatigue, though not the kind of symptoms typically associated with the coronavirus.
If it is true that asymptomatic or minimally symptomatic people can transmit the disease frequently and efficiently, testing may need to be broadened, experts said.
"This implies we may need many more tests that can be used out in the field, at the point of care," said Dr. Judith N. Wasserheit, co-director of the University of Washington MetaCenter for Pandemic Preparedness and Global Health Security. "We're still learning about the biology of this virus and how it causes disease."
Dr. Sandra Ciesek, of the Institute of Medical Virology at University Hospital Frankfurt, who was one of the authors of a letter in The New England Journal of Medicine that described the German patients who did not become ill, said the problem was that "normally, you don't screen asymptomatic healthy people for the virus because it's too expensive."
"This shows we might have more infected people already all over the world than we expect," she said.