A nurse who treated an Ebola-stricken Liberian man at a Dallas hospital has contracted the disease despite wearing extensive protective gear, officials said Sunday in announcing the first apparent transmission of the deadly virus in the United States.

Word of the infected American health-care worker came four days after the death of Thomas Eric Duncan, who died Wednesday at Texas Health Presbyterian Hospital Dallas after battling Ebola there for more than a week.

Thomas Frieden, director of the Centers for Disease Control and Prevention, said the health-care worker, who was not identified but whom others have said is a female nurse, had “extensive contacts” with Duncan on “multiple occasions.” Still, he and other authorities said it remains unclear what sort of safety breach led to the infection.

The case raises new doubts about whether hospitals around the country, aside from a handful of highly-specialized facilities, are truly prepared to safely deal with the Ebola virus, and whether front-line nurses and doctors have received adequate training in diagnosing and treating the disease.

The incident also seems certain to intensify fears about how easily the disease can spread, even though Ebola is transmitted only through bodily fluids and only after a patient begins showing symptoms.

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The Dallas nurse had not been considered a “high risk” contact of Duncan’s, but rather was one of 18 employees the hospital was monitoring for potential symptoms. The CDC is monitoring another 48 people who had contact with Duncan before he was admitted to the hospital, including his relatives.

“We’d known that further cases of Ebola are a possibility for those who were in close contact” with Duncan, said Daniel Varga, chief clinical officer for Texas Health Resources, which operates the hospital in Dallas.

Officials said the nurse had been under “self-monitoring” at home in recent days, which includes twice-daily temperature checks. On Friday, she reported a “low-grade fever” and was referred for testing. Varga said the nurse notified the hospital after showing symptoms, came directly there and was admitted to an isolation room. He said the entire sequence of events took less than 90 minutes.

A state laboratory delivered a preliminary Ebola diagnosis late Saturday. The CDC confirmed those results Sunday afternoon.

Local, state and federal officials rushed on Sunday to calm fears among Dallas residents and people throughout the country, saying they always expected more Ebola cases could surface among people who encountered Duncan, but quickly isolating those patients and monitoring their contacts would halt a broader spread of the disease.

“While this is obviously bad news, it is not news that should bring about panic,” Dallas County Judge Clay Jenkins said during an early-morning news conference.

Dallas officials deployed hazmat teams to decontaminate the entrance and common areas of an apartment complex on Marquita Avenue where the health worker lives, as well as the vehicle the nurse used to travel to the hospital. The person’s residence is under police guard, and a hazmat team was scheduled to decontaminate it later Sunday. 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.

Dallas Mayor Mike Rawlings also said officials have a plan to take care of a dog living in the worker’s apartment. It had shown no symptoms of the disease.

Varga added that hospital officials, while “very concerned” about the nurse’s infection, remain “confident that the precautions that we have in place are protecting our health-care workers.”

Others aren’t so sure.

“We are seeing that hospitals are not prepared,” said Bonnie Castillo, director of the Registered Nurse Response Network, a part of National Nurses United, the nation’s largest nurses union. “They are not doing active drilling and education they need to be doing.”

Castillo pointed to a survey the group recently conducted of nurses across the country, which showed that the vast majority believe they have not received adequate training on Ebola protocols.

“I’m fearful for my nurse colleagues who are putting it on the line when they walk through hospital doors,” Castillo said. “They walk through those doors every day, and they know they could be exposed to infectious disease. The hospitals are not providing them with measures they need.”

Hospital officials said the infected nurse was “following full CDC precautions” — including wearing a gown, gloves, a mask and a protective face shield — when she began treating Duncan upon his second visit to the hospital on Sept. 28. He had been turned away days earlier, despite having a fever and disclosing that he recently had traveled from Liberia.

But even for those trained in working under such circumstances, correctly putting on and taking off protective gear is time-consuming and tricky.

“It’s not easy to do right,” Frieden said. He noted that removing personal protective equipment, in particular, presents one of the biggest risks for infection. According to CDC guidelines, protective equipment must be removed in a precise order, without any of the material touching the wearer’s skin, mucus membranes or the exterior of the clothing.

One version of the process begins with removing 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.

For instance, a Spanish health worker who became infected after treating an Ebola-stricken priest in Madrid possibly contracted the virus while removing her protective suit. She recalled possibly touching her face with her glove, a simple slip that could have opened the door to infection.

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.

President Obama was briefed Sunday morning on the situation in Dallas and directed the CDC to conduct a probe into the apparent breach of infection control protocols there “as expeditiously as possible,” the White House said. He also ordered federal officials to “take immediate additional steps” to ensure hospitals and health-care providers across the country are prepared to handle suspected Ebola cases.

Frieden said Sunday that the CDC is considering treating any Ebola patients — he underscored that more infections could happen — at one of four facilities in the United States that have specialized 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 in Omaha — already have treated confirmed or suspected Ebola cases. The fourth place is St. Patrick Hospital in Missoula, Mont. But at the same time, he said, “We can’t let any hospital let its guard down.”

The CDC is recommending that hospitals caring for Ebola patients keep the number of staff to a minimum and that a full-time staffer supervise and monitor infection control. That means knowing how to properly use protective equipment.

“Putting more on isn’t always safer,” Frieden said.

As the Ebola epidemic continues to grow virtually unchecked in West Africa, the odds increase that the disease will spread to other countries. The outbreak has infected nearly 8,400 people and killed more than 4,000 in the hardest-hit countries of Liberia, Sierra Leone and Guinea, according to the World Health Organization.

In the meantime, Frieden said the CDC is sending additional employees to Dallas and already has begun tracking all the contacts the health worker could have had since developing symptoms, just as it did with Duncan.

The nurse remained in isolation and in stable condition on Sunday, officials said. The hospital has put its emergency room on “diversion,” which means that ambulances are not bringing patients to its emergency room, though patients already in the hospital are still being cared for.

Duncan, the first person diagnosed with Ebola in the United States, made stops in Brussels and at Dulles International Airport near Washington on his way to Texas, though he showed no symptoms until days after his arrival. Before his trip, he reportedly had helped carry a sick pregnant woman to a taxi in Liberia and rode with her while she sought medical care. She later died of Ebola.

On Sunday, amid the fear and frustration and unanswered questions, Dallas County Judge Jenkins said the latest diagnosis underscores one of the virus’s cruelties: That those caring for Ebola patients are the ones most at risk of becoming Ebola patients.

“That health-care worker is a heroic person,” Jenkins said. “Let’s remember, as we do our work, this is a real person who is going through a great ordeal.”

J. Freedom du Lac, DeNeen Brown, Amy Ellis Nutt and Juliet Eilperin contributed to this report.