The nurse, Amber Joy Vinson, 29, was on the medical team that cared for the Ebola victim Thomas Eric Duncan after he was admitted to the hospital on Sept. 28 and put in isolation. Ms. Vinson should not have traveled on a commercial flight, federal health officials said, when she boarded Frontier Airlines Flight 1143 on Monday, en route from Cleveland to Dallas-Fort Worth.

One official said Ms. Vinson had called federal health officials before boarding the plane to report having a slightly elevated temperature but was allowed to fly.

A second case of Ebola among the nearly 100 doctors, nurses and assistants who treated Mr. Duncan for 10 days at Texas Health Presbyterian Hospital was not unexpected. For days, federal health officials have warned that in addition to Nina Pham, the first nurse confirmed with the disease, other cases were likely.

President Obama on Wednesday canceled a trip to a fund-raiser in New Jersey and a campaign rally in Connecticut so he could convene a meeting of officials to coordinate the government's response to the spread of the Ebola virus. Cities and states adopted heightened security measures, and Ms. Vinson was being treated at Emory University Hospital in Atlanta rather than in Dallas — all signs of the heightened focus on the disease and the fears it has stirred.

Speaker John A. Boehner of the House on Wednesday became the highest-profile Republican to urge Mr. Obama to consider a ban on travel to the United States from West African countries where the Ebola virus is spreading rapidly. The same point has been raised by several Republican candidates in tight Senate races, including David Perdue of Georgia, Mike Rounds of South Dakota and Joni Ernst of Iowa.

At Kent State University in Ohio, where Ms. Vinson studied nursing and her mother and two other relatives worked, officials issued a statement asking her three family members to stay off campus for the next three weeks, "out of an abundance of caution." Frontier Airlines said it had put the two pilots and four flight attendants on Flight 1143 on paid leave.

The Frontier jet that carried Ms. Vinson on Monday made five flights after her trip before it was taken out of service, according to Flightaware.com. Frontier, which is based in Denver, said it grounded the plane as soon as it was notified, about 1 a.m. Wednesday, about the Ebola patient.

The first nurse infected, Ms. Pham, 26, remained in good condition, the hospital said. Ms. Vinson was ill but stable, as she was transferred Wednesday to Emory, one of four hospitals in the United States that have special containment units for isolating patients with dangerous infectious diseases.

Though Ms. Vinson had traveled on Monday, the day before she reported symptoms, she had been among a group of hospital workers at Presbyterian who were being monitored after Ebola was diagnosed on Sunday in Ms. Pham. And although her temperature did not meet the fever threshold of 100.4, Ms. Vinson reported to health officials that her temperature at the time she traveled was 99.5 degrees.

"Because at that point she was in a group of individuals known to have exposure to Ebola, she should not have traveled on a commercial airline," the C.D.C. director, Dr. Thomas R. Frieden, said. "The C.D.C. guidance in this setting outlines the need for what is called ‘controlled movement.' That can include a charter plane, that can include a car but it does not include public transport." Hours after Dr. Frieden spoke, a federal health official who spoke on condition of anonymity because he was not authorized to speak publicly, said that because it was thought that Ms. Vinson's protective gear would have kept her safe and because the temperature was only mildly elevated, she fell into a category not covered by C.D.C. guidelines.

"I don't think we actually said she could fly, but they didn't tell her she couldn't fly," the official said. He said the error was on the part of the C.D.C., not the nurse. "She called us," he said. "I really think this one is on us."

Because of the proximity between the evening flight Monday and her first report of being ill on Tuesday morning, the C.D.C. asked all 132 passengers on the flight to call a C.D.C. hotline. The flight landed at 8:16 p.m. Monday at Dallas-Fort Worth International Airport. Dr. Frieden stressed that the passengers were a low-risk group. Because Ms. Vinson did not have a fever and did not experience nausea or vomiting on the plane, the risk "to any around that individual on the plane would have been extremely low," he said.