“She should not have flown on a commercial airline,” Frieden said.

This health-care worker flew on a Frontier Airlines flight from Cleveland to Dallas-Fort Worth with more than 130 other passengers. She did not have nausea or vomit on the plane, so the risk to anyone around her is “extremely low,” Frieden said.

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The health-care worker was not named by public health officials, but a spokesman for Cleveland identified her Amber Vinson. Family members told Reuters and the Dallas Morning News that Vinson is a nurse at Texas Health Presbyterian Hospital. She was part of a team that had cared for Thomas Eric Duncan, a Liberian man who flew to Texas and was diagnosed with Ebola last month, during his hospitalization in Dallas. Duncan died last week. Nina Pham, a nurse who also cared for Duncan, was diagnosed with Ebola on Sunday and was in good condition Wednesday, the hospital said.

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Vinson, who flew from Dallas to Cleveland on Friday, flew back to Texas on Monday, a day after Pham was diagnosed. She reported a fever on Tuesday and was isolated and tested for Ebola.

Still, the fact that she boarded a commercial flight raises the question of how much the other 50 health-care workers who entered Duncan’s room could have traveled or moved around in recent days. The CDC recommends controlled movement on private flights or vehicles for people who may have been exposed to Ebola, Frieden said.

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“We will, from this moment forward, ensure that no individual monitored for exposure undergoes travel in any way other than controlled movement,” Frieden said Wednesday. He said the agency would work with state and local authorities to enforce this restriction.

It is still unclear how, exactly, Pham and Vinson were infected with Ebola, but Frieden suggested on Wednesday that it occurred during the days after Duncan was admitted to the hospital and before the CDC team arrived. Duncan was placed in isolation at the hospital on Sunday, Sept. 28, and the CDC did not arrive until Tuesday, Sept. 30, the day Duncan was diagnosed. Pham and Vinson both cared for Duncan during these days and had “extensive contact” with Duncan, who was vomiting and had diarrhea, Frieden said.

Vinson, who was being treated at Texas Health Presbyterian, will be transferred Emory University Hospital in Atlanta on Wednesday for treatment, according to the CDC. She is “ill but clinically stable,” Frieden said.

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While visiting Ohio, she stayed with three family members who are employees of Kent State University, the school said in a statement Wednesday. She did not actually visit the campus, Kent State President Beverly Warren said in the school’s statement. The three family members have been asked to stay off of the school’s campus and self-monitor for the next 21 days.

Vinson graduated from Kent State, earning degrees there in 2006 and 2008, according to the university.

Even though the risk remains low in Ohio, the state Department of Health has epidemiologists working with local authorities in Summit County, where Vinson stayed, according to Ohio Gov. John R. Kasich (R).

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“Ohio has a sophisticated state and local public health network that has been preparing for this possibility for several months and those plans are now being activated,” Kasich said in a statement.

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Health officials in Summit County said during a briefing Wednesday that they were still working to construct a timeline of Vinson’s travels in the state.

The plane Vinson traveled on arrived in Dallas at 8:16 p.m. on Monday and remained overnight because it was done with flights for the day. After that, the plane “received a thorough cleaning per our normal procedures” before resuming service Tuesday, Frontier said in a statement. It was also cleaned on Tuesday night in Cleveland, the airline said.

Ricky Smith, director of Cleveland’s Hopkins International Airport, said the plane Vinson flew will make a scheduled trip to Denver.

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“At this point we have no reason to believe those facilities were contaminated or infections,” Smith said. “We are taking precautionary measures.”

People with Ebola are not contagious until they are symptomatic, public health officials say. Authorities had said that Vinson was not exhibiting any symptoms until Tuesday, though Frieden later said she had a slight fever on Monday. Ebola can be spread through direct contact with bodily fluids, and it is considered difficult to contract.

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The CDC is still asking all 132 passengers on Frontier Flight 1143 to call 800-232-4636, a hotline that will put them in touch with public health officials to determine if any of them need to be monitored going forward. But it is not known how many crew members were on the flight as well as how many people were involved in the cleaning of the plane in Dallas and Cleveland.

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“The safety and security of our customers and employees is our primary concern,” Frontier said in a statement Wednesday. “Frontier will continue to work closely with CDC and other governmental agencies to ensure proper protocols and procedures are being followed.”

The Service Employees International Union, which represents many hospital workers and the crews assigned to clean airplanes between flights, reiterated its concerns about safety on Wednesday.

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“We must ensure that working women and men who are on the front lines of protecting our communities have the necessary training and support that will enable them to safely and effectively limit the risks of Ebola exposure,” said SEIU president Mary Kay Henry.

Thomas Geisbert, a professor at the University of Texas Medical Branch at Galveston who has researched the Ebola virus for decades, said it isn’t entirely clear when an infected person actually becomes symptomatic. “We don’t know with 100 percent certainty” when an infected person begins shedding the virus, Geisbert said. “People go out and say things that are black and white, and they are getting themselves in trouble.” He agreed that, typically, patients become contagious only after developing symptoms, and as the level of virus mounts in their blood and other bodily fluids. But he there have been anecdotal cases in the past of “super shedders” – people who, for some genetic reason, give off much more of the virus than the average patient. “It’s okay to talk in general … [But] you have to be very careful,” Geisbert said. “There are always outliers.” Dallas County Judge Clay Jenkins said at an early-morning news conference that Vinson was isolated within 90 minutes of having a higher temperature. He had also warned that additional cases of Ebola were considered likely. “We are preparing contingencies for more, and that is a very real possibility,” Jenkins said. President Obama postponed a trip to New Jersey and Connecticut Wednesday to

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with cabinet agencies to discuss the Ebola response. “What we’ve been doing here today is reviewing exactly what we know about what’s happened in Dallas and how we’re going to make sure that something like this is not repeated,” Obama said

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. Obama stressed that the odds of a widespread outbreak are “very, very low,” but said the country would be reviewing its screening protocols at U.S. airports and work to inform the public about what is happening around the country. “We are going to be monitoring carefully the health status of the other health-care workers in Dallas,” he said. “Obviously, they’re concerned. We understand that many of them are scared. We are going to make sure that we are on the ground 24/7 to provide them the kind of support, information and assurances that they need to get through this particular challenge.” The CDC had said Tuesday

how Pham was infected, but it had said a team of about 76 people — including Vinson — had possible exposure to Duncan during his care. On Wednesday, Frieden said about 50 health-care workers entered Duncan’s room, but it was not immediately known if the other people were still viewed as at risk.

“A lot is being said about what may or may not have occurred to cause our colleagues to contract this disease, but it’s clear there was an exposure somewhere, sometime in their treatment of Mr. Duncan,” Daniel Varga, chief clinical officer for Texas Health Resources, the hospital system that oversees Texas Health Presbyterian, said Wednesday. “We’re a hospital that may have done some things differently with the benefit of what we know today.”

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National Nurses United, a labor organization, issued a statement Tuesday night it said came from nurses at the hospital who reported feeling “unsupported, unprepared, lied to and deserted to handle their own situation.”

Among other things, the statement said that the guidelines for handling Duncan were constantly shifting, and that for the first two days of Duncan’s isolation, the protective equipment given to nurses did not cover their necks.

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

Frieden had said Tuesday that workers at hospitals across the United States were extremely concerned about treating Ebola patients.

“I’ve been hearing loud and clear from health care workers around the country that they’re worried, that they don’t feel prepared to take care of a patient with Ebola,” he said during a news conference.

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Frieden expressed regret that his agency had not done more to help the hospital in Dallas with infection control. He said that from now on, the CDC will dispatch an “Ebola response team” to any hospital with a confirmed Ebola case.

“I wish we had put a team like this on the ground the day the first patient was diagnosed,” he said. “That might have prevented this infection.”

The CDC sent an additional team of 16 people to Texas to help manage infection control and monitor hospital workers, following an initial group of 10 that had been sent on Sept. 30, the day Duncan was diagnosed.

The infections of two health-care workers in Dallas, combined with the numerous times the hospital has changed its story regarding the way Duncan was initially cared for, have created a sense of unease about how prepared U.S. hospitals are for additional Ebola cases.

[This post has been updated. First published: 11:28 a.m. Last update: 5:51 p.m.]

Abby Phillip, Brady Dennis, Ashley Halsey III, Elahe Izadi and Abby Ohlheiser contributed to this report.