DALLAS — In Washington, a patient who had traveled to Nigeria and who was suspected of having Ebola was placed in isolation at Howard University Hospital on Thursday. In New Haven, two Yale University graduate students plan to sequester themselves when they return this weekend from Liberia, where they have helped the government develop a system to track the Ebola epidemic.

And at Newark Liberty International Airport on Saturday, a sick man who had just arrived from Brussels was rushed to a hospital amid concerns that he was showing Ebola-like symptoms, a fear later dismissed by the Centers for Disease Control and Prevention.

With fears about Ebola widening across the United States, federal health officials said Saturday that they were receiving an escalating number of reports of possible Ebola infection, particularly after a Liberian man tested positive for the deadly disease in Dallas last week, the first Ebola case diagnosed in this country. Since the disease began spreading rapidly across West Africa this summer, the C.D.C. said, it has assessed more than 100 possible cases, but only the Dallas case has been confirmed.

But increased attention about the virus has jangled nerves around the country, particularly among West African immigrant communities and recent travelers to that region, and placed health care workers on a kind of high alert. “We expect that we will see more rumors, or concerns, or possibilities of cases,” Dr. Thomas R. Frieden, director of the federal C.D.C., said Saturday. “Until there is a positive laboratory test, that is what they are — rumors and concerns.”

Image Dr. Thomas R. Frieden, director of the Centers for Disease Control and Prevention. Credit... Tami Chappell/Reuters

In a sign of the seriousness of the virus, the Dallas hospital where the Liberian man, Thomas E. Duncan, has been recovering changed the status of his condition on Saturday from serious to critical.

In the more than 100 inquiries the C.D.C. has received about possible Ebola, about 15 people were actually tested for the virus, officials at the disease centers said. In addition to doing their own testing on suspected cases, federal officials have helped more than a dozen laboratories around the United States do Ebola testing.

One of those cases was at Howard University Hospital, which said Saturday that it had “ruled out” Ebola in a patient who was admitted on Thursday. The patient, who had traveled to Nigeria, had been placed in isolation “in an abundance of caution,” said a statement by the university’s president, Dr. Wayne A. I. Frederick.

The Newark airport case only added to the heightened nervousness. After United Airlines Flight 998 from Brussels arrived in Newark shortly after noon, officials from the C.D.C. took a 35-year-old man who had been vomiting to University Hospital in Newark “as a precaution,” said Erica Dumas, a spokeswoman for the Port Authority of New York and New Jersey. The flight had 255 passengers and 14 crew members on board, the airline said.

Hospital officials later said that the man had symptoms “consistent with another, minor treatable condition unrelated to Ebola.” A child accompanying him had no symptoms. Both were released and will continue to be monitored, the hospital said in a statement.