A man who took a commercial flight from Liberia that landed in Dallas on Sept. 20 has been found to have the Ebola virus, the Centers for Disease Control and Prevention reported on Tuesday. He is the first traveler to have brought the virus to the United States on a passenger plane and the first in whom Ebola has been diagnosed outside of Africa in the current outbreak.

As the disease has swept across West Africa, many health experts said it would be only a matter of time before it reached the United States. Hospitals and health departments around the country have been preparing for it, and a number of false alarms have occurred. But this time, the case is real.

The man, who was visiting relatives in the United States, was not ill during the flight, health officials said at a news conference Tuesday evening. Indeed, he was screened before he boarded the flight and had no fever. Because Ebola is not contagious until symptoms develop, there is “zero chance” that the patient infected anyone else on the flight, said Dr. Thomas R. Frieden, director of the disease centers. Ebola is spread only by direct contact with body fluids from someone who is ill.

A team from the C.D.C. is being dispatched to Dallas to help trace any contacts who may have been infected, including family members, health care workers and others with whom the patient spent time in Dallas. Health officials in Texas said they had already begun that process. Dr. Frieden said the family and community contacts were few, no more than a handful. But he said it was possible that family members who were with the man while he was ill would turn out to be infected.