Federal health officials warned on 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,” Dr. Fauci said.

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.