Thomas Frieden, director of the CDC, said that an unknown breach in protocol led to the Texas Health Presbyterian Hospital worker being infected and that federal officials are investigating. He said additional CDC staff are heading to Texas.

Frieden also said more cases may be likely.

“We are deeply concerned by the news,” he said during a news conference Sunday. Frieden said the worker, whom other officials identified as a female nurse, provided care for Duncan on “multiple occasions” that included “extensive contacts with him.”

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Frieden also said the CDC is considering having Ebola patients be treated at one of the four facilities in the United States that have special isolation units. Three of them — the National Institutes of Health in Bethesda, Md.; Emory University Hospital in Atlanta; and the University of Nebraska Medical Center — have treated confirmed or suspected Ebola cases. The fourth place is St. Patrick Hospital in Missoula, Mont.

He outlined several steps that were being taken to care for the health-care worker and prevent the infection of others. Every effort is being made to care for the patient safely and effectively, he said.

The Texas Health Presbyterian Hospital worker reported “a low-grade fever” Friday, according to the Texas Department of State Health Services. She “was isolated and referred for testing,” and the preliminary test result was received late Saturday. The CDC confirmed the test on Sunday afternoon.

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The CDC did not consider the nurse to be “high risk,” said Daniel Varga, chief clinical officer for Texas Health Resources, which operates Texas Health Presbyterian Hospital in Dallas. She treated Duncan, the Ebola patient, after his second visit to the emergency room, on Sept. 28, and was “following full CDC precautions,” including wearing a gown, gloves, a mask and a protective face shield.

“We’re very concerned,” Varga said, though he added that the hospital is “confident that the precautions that we have in place are protecting our health-care workers.”

Taking off personal protective equipment — gowns, gloves, face masks, goggles — is also one of the biggest areas of contamination and risk, Frieden said. According to CDC guidelines, protective equipment must be removed in a particular order, without any of the material touching the wearer’s skin, mucus membranes or the exterior of the clothing.

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One version of the process begins with removing infected gloves, then the goggles or face shield, followed by the protective gown, which must be removed from the inside out, and finally the face mask or respirator. If hands become contaminated by touching the outside of the equipment, they must be washed after each step in the process of removing the personal protective gear.

“It’s not easy to do right,” Frieden said. CDC and state officials will be investigating how hospital workers followed recommended protocols for doing so.

Officials are also assessing all possible contacts the health-care worker may have had from the moment she developed symptoms on Friday. A CDC epidemiologist has interviewed her, and, so far, there is only one other person who may have had contact with the worker since the symptoms developed, and that individual is under active monitoring, Frieden said.

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State, local and federal officials are trying to identify the number of other health-care workers who may potentially have been at risk of exposure, he said. Earlier Sunday, a hospital official said the hospital is tracking the condition of 18 hospital employees. Frieden clarified that the 48 other people who have been monitored — and are not showing symptoms — were people who may have come into contact with Duncan before Sept. 28, when he was admitted to the hospital. Now that the health-care worker is being treated, officials are determining how many other health-care workers may have been exposed after Sept. 28.

It was unclear what role the nurse had in caring for Duncan, who was the first person in the United States to be diagnosed with Ebola.

Dallas County Judge Clay Jenkins called the nurse “heroic” for her efforts to care for Duncan, and he added that Dallas officials anticipated that a second person could contract the virus.

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The nurse is in isolation and in stable condition, the hospital system said.

James Wortman, 83, a neighbor of the Ebola-stricken nurse, said he watched the news on Dallas television.

“It’s got me shook,” said Wortman, who said he has spent the last few months away from his home caring for his wife, who suffered a massive stroke. “I know the nurses aides in the rehab center where my wife is, they are concerned about it. They are just talking about the news.”

Varga said the nurse had been under self-monitoring in recent days, which includes taking a temperature twice daily. When she showed signs of a fever on Friday, she notified the hospital, went directly there and was immediately admitted to an isolation room. Varga said the entire sequence of events took less than 90 minutes.

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Despite transmission of the disease to the health worker, Anthony Fauci, acting head of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, said the system for preventing spread of the virus has long worked for organizations that frequently deal with Ebola patients, such as Doctors Without Borders.

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“She was on voluntary self-monitoring,” Fauci said. “She found she got infected, and she immediately did what she was supposed to have done. So even in this troublesome situation, the system is working.”

The hospital has put its emergency room on “diversion,” which means that ambulances are not currently bringing patients to the ER, though patients already in the hospital are still being cared for.

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Frieden said federal officials are working with the hospital to “ramp up training and education.” He noted that caring for Ebola patients requires “meticulous and scrupulous attention to infection control” and that an “inadvertent slip” can result in contamination and infection.

Going forward, the CDC is recommending that hospitals keep the number of staff caring for Ebola patients to a minimum and that medical procedures also be kept to a minimum. In addition, a full-time individual should supervise and monitor infection control while Ebola patients are being cared for and review the appropriate use of personal protective equipment. “Putting more on isn’t always safer,” he said.

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For medical procedures, for example, if a patient is not having severe symptoms such as vomiting and diarrhea, it may be more prudent to draw the patient’s blood to check for electrolyte levels once a day instead of multiple times a day, to minimize risk.

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Unlike the several other patients who have been treated for Ebola in U.S. hospitals, Duncan underwent more high-risk medical procedures, including kidney dialysis and intubation — a tube to help him breathe — as “desperate measures to try to save his life.”

“Both those procedures spread many contaminants and are high risk,” Frieden said.

Dallas officials deployed hazmat teams to decontaminate the entrance and common areas of an apartment complex on Marquita Avenue where the health worker lives and the vehicle that the person used to travel to the hospital. That person’s home has been secured, and law enforcement officials are ensuring that no one enters. The city also knocked on doors and issued reverse 911 calls to homes in the area and distributed information sheets to homes in the area on Sunday morning.

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Dallas Mayor Mike Rawlings also said that there is a pet in the worker’s apartment and that “we have a plan in place to take care of the pet,” which seems to have shown no signs of the disease. He said the hazmat team plans to enter the apartment later Sunday and decontaminate it.

President Obama was briefed Sunday morning on the diagnosis of a second Ebola case in Dallas, and he later discussed the situation with Health and Human Services Secretary Sylvia Burwell.

Obama directed the CDC to conduct its probe of the apparent breach of infection control protocols there “as expeditiously as possible,” the White House said, and share the lessons it learns from the incident “quickly and broadly.” He also ordered federal officials to “take immediate additional steps” to ensure hospitals and health-care providers across the country are prepared to follow protocols if they need to treat an Ebola patient.

The initial Ebola patient, Thomas Duncan, traveled by plane from Liberia to Texas through Brussels and Dulles International Airport near Washington, though he wasn’t symptomatic at the time.

He became sick several days after arriving in Dallas and first sought treatment Sept. 25. But he was released by the hospital, despite saying that he had traveled from Liberia and that he had a fever and abdominal pain.

He was admitted to Texas Health Presbyterian Hospital on Sept. 28, and the CDC confirmed Sept. 30 that he was infected with Ebola. He died on Oct. 8.

Ebola is contagious only if someone has symptoms. It can spread through bodily fluids or infected animals but not through the air.

The virus has killed more than 4,000 people and infected more than twice as many this year, according to the World Health Organization.

Months after the deadliest Ebola outbreak in history began ravaging West African countries, the virus has slowly begun to spread around the world.

On Monday, a Spanish sanitary technician who had treated an Ebola-stricken priest in Madrid tested positive for the virus, becoming the first case of Ebola transmission outside Africa.

The nursing assistant in Madrid said she may have contracted the virus while removing her protective suit. Health-care workers at her hospital are now refusing to work, out of concern that the safety conditions there are inadequate.

Sebastian Payne, DeNeen L. Brown, Juliet Eilperin, Josh Hicks and J. Freedom du Lac contributed to this report.