The experts had warned that fighting Ebola is hard, and Wednesday’s drumbeat of bad news proved them correct. The day began with a bulletin about another health-care worker stricken with the deadly disease, and the news got worse with the revelation that she had flown with a slightly elevated temperature from Cleveland to Dallas on a crowded airliner barely 24 hours before her diagnosis.

Before she boarded that flight, the woman, identified by Ohio officials as Amber Joy Vinson, 29, informed the Centers for Disease Control and Prevention that she was running a temperature of 99.5 degrees, a federal official told The Washington Post.

That was below the 100.4-degree­ threshold in CDC guidelines for screening travelers who have been in Ebola-affected countries, and which triggers a secondary screening. The CDC did not prohibit Vinson from traveling on the plane back to Dallas, said the official, who spoke on the condition of anonymity because of the sensitive nature of the issue.

But on Wednesday, CDC Director Thomas Frieden said that Vinson should not have been flying anywhere given her possible exposure to Ebola at her workplace, Texas Health Presbyterian Hospital Dallas, which has been the epicenter of the crisis in the United­ States.

Vinson did not yet have the classic symptoms of a full-blown Ebola infection, such as vomiting and diarrhea, and so epidemiologists doubt that she spread the virus during the journey. Ebola is spread through direct contact with bodily fluids, which carry a higher viral load as the disease progresses.

But Frontier Airlines and the CDC scrambled to contact the 132 passengers aboard Flight 1143, and the people on that flight joined a growing pool of people in the United States who may have been exposed to the virus in recent weeks.

As Wednesday ended, Americans had to be wondering when the U.S. outbreak will be contained, and whether public officials’ measured language and repeated reassurances are a gloss on a desperate and sometimes improvisational battle against a disease that in West Africa has killed more than 4,000 people.

The Vinson case highlighted how easily someone who is unknowingly infected can travel a great distance and potentially expose hundreds of new people. Scores of hospital staffers were involved in the treatment of the “index patient,” Thomas Eric Duncan, the Liberian man who died of Ebola at the hospital Oct. 8.

It is now clear that Presbyterian Hospital experienced a catastrophic failure of infection control when it treated Duncan. He fell ill four days after arriving in Dallas by plane from Liberia, a trip that included connecting flights in Brussels and at Washington Dulles International Airport. When Duncan first went to the hospital, he was sent home despite a high fever and his stated travel history, a misstep that the hospital still has not fully explained. Two days later, on Sept. 28, he returned, with his family fearing that he had Ebola.

Duncan, according to medical records obtained by the Associated Press, suffered extreme diarrhea and projectile vomiting in the ensuing hours. Two of the health-care workers who treated him in those first days are now sick with Ebola: Nina Pham, a nurse, who is listed in good condition at the hospital, and Vinson, who, according to a review of medical records by the AP, inserted catheters, drew blood and dealt with bodily fluids.

Not until Sept. 30 did tests confirm that Duncan had Ebola. During those first three days, the health-care workers at the hospital did not have full biohazard protection from head to toe, but rather used more conventional protective gear, supplemented with additional layers, according to Frieden. Some, fearful of the contagion, wore three or four layers of protective equipment in the mistaken belief that more was better in such a situation, he said.

“By putting on more layers of gloves or other protective clothing, it becomes harder to put them on and take them off,” he said.

Vulnerabilities in Ebola planning

The CDC has sent a “go team” to offer counsel on infection-control practices at Presbyterian Hospital, and it said it will send a similar team within hours to any hospital that admits an Ebola patient, but that announcement came two weeks after Duncan’s diagnosis. Vinson was transferred Wednesday evening to a special isolation unit at Emory University Hospital in Atlanta.

That hospital has had success treating several Ebola patients during this outbreak, including one, an anonymous patient who became sick in West Africa and who said, in a statement released Wednesday by the hospital: “Through rigorous medical treatment, skillful nursing, and the full support of a healthcare team, I am well on the way to a full recovery. I want the public to know that although Ebola is a serious, complex disease, it is possible to recover and return to a healthy life.”

The mounting crisis has kicked up political sparks. Some lawmakers, including House Speaker John A. Boehner (R-Ohio), called for a ban on travelers from countries affected by the outbreak. President Obama canceled a trip Wednesday and convened a meeting of his Cabinet to discuss the Ebola threat, pledging afterward “a much more aggressive” response to the situation in Dallas. The president, who also has canceled a trip Thursday, tried to assure the American people that they won’t get Ebola, citing his own visit to Emory University Hospital:

“I shook hands with, hugged and kissed not the doctors, but a couple of the nurses at Emory, because of the valiant work that they did in treating one of the patients. They followed the protocols, they knew what they were doing, and I felt perfectly safe doing so,” Obama said.

He also spoke in a videoconference with the leaders of Britain, Italy, Germany and France, again urging a more concerted global response to the epidemic in West Africa. The U.S. military continues to add to the force it is deploying there, but it will take until late November or early December to complete all 17 of the 100-bed treatment units it has planned, said Maj. Gen. Darryl A. Williams, the commander of U.S. Army Africa.

Obama, Frieden and other officials have stressed that Ebola is unlikely to become an epidemic here the way it has in West Africa. But health-care workers have repeatedly expressed their concerns to the CDC in recent days.

“We’ve been lied to in terms of the preparation in the hospitals,” said RoseAnn DeMoro, executive director of National Nurses United. “What happened in Dallas can happen anywhere.”

Frieden said Wednesday that about 50 health-care workers entered Duncan’s hospital room. He said that henceforth they will be required to limit their travel to certain safe methods, which do not include flying on commercial aircraft.

Officials have not said how many crew members were on the Frontier flight from Cleveland to Dallas, nor have they said how many people were involved in cleaning the plane. The aircraft arrived in Dallas at 8:16 p.m. Monday and remained overnight before resuming service the following day. It was cleaned in the interim and was cleaned again Tuesday night in Cleveland, the airline said.

While visiting Ohio, Vinson stayed with three family members who are employees of Kent State University, the school said in a statement Wednesday. She did not visit the campus, Kent State President Beverly Warren said. The three family members have been asked to stay off the school’s campus and monitor themselves for the next 21 days. Vinson earned degrees from Kent State in 2006 and 2008, according to the university.

Dallas County Judge Clay Jenkins said at an early-morning news conference that Vinson was isolated within 90 minutes of reporting a higher temperature. He warned that additional cases of Ebola were considered likely.

“We are preparing contingencies for more, and that is a very real possibility,” Jenkins said.

Sun reported from Atlanta. Elahe Izadi, Abby Phillip, Juliet Eilperin, Brady Dennis, Amy Ellis Nutt, Katie Zezima and Dan Lamothe in Washington contributed to this report.