The first person diagnosed with Ebola in the United States boarded commercial jets that took him to two of the country’s busiest airports. He flew from Liberia to Brussels before boarding a United Airlines jet to Dulles International Airport outside Washington, where he changed planes and boarded another United Airlines jet to Dallas-Fort Worth.

This is obviously worrisome to many people, as it involves not just the arrival of a person diagnosed with Ebola in the country but our larger fear of contagion, outbreak and a disease on the move. Airplanes are not exactly the cleanest of places, and we board them not knowing whom we are sitting next to or where they have been. That has resulted in fearful reporting like this, which frantically tracked all of the cities where these planes have gone since carrying Thomas Duncan. They went to San Francisco! And Chicago! And Los Angeles! They’re right behind you, watching you read this post.

In reality, though, officials continue to say that there is no danger posed by these flights or by Duncan’s presence in the airports. Here’s why: Duncan, who was diagnosed with Ebola on Tuesday, arrived in the country on Sept. 20. He did not start showing any symptoms until four days later, according to the Centers for Disease Control and Prevention. And Ebola is contagious only when the person is experiencing active symptoms.

Duncan’s temperature was checked before he boarded the plane in Liberia, when he was not showing any symptoms, CDC Director Thomas Frieden said during a news conference earlier this week.

“There is zero risk of transmission on the flight,” Frieden said.

United Airlines has released the flight numbers and paths for the trips Duncan took to Dulles and Dallas on Sept. 20. He was on Flight 951 from Brussels to Dulles and then changed planes, boarding Flight 822 to Dallas.

“While the CDC states it is unnecessary for it or the airline to contact others who were on the patient’s flights, United is providing information about the flights United believes the patient took, based on information provided by the CDC,” the airline said in a statement. “We are ensuring our employees have this information and suggest that any customers who have concerns contact the experts at the CDC for further information.”

Airlines have also worked to tamp down concerns that commercial jets could be used to carry the disease in the U.S.

The screening at the airport in Monrovia, the capital, has been in place for months, Deborah R. Malac, the U.S. ambassador to Liberia, told The Washington Post in a telephone interview Thursday.

“They have confidence that everything that was supposed to have been done was done,” Malac said.

The State Department has warned U.S. citizens against “non-essential travel to Liberia.” There are no plans at this point to update this policy in the wake of Duncan’s diagnosis, a department official said.

Experts have been saying for months that the risk faced by people in the United States is incredibly low. They have reiterated for months — up to and including this week — that the U.S. health-care system and infrastructure are different than those in West Africa.

Kevin Sieff contributed to this report.