WASHINGTON — A breach in safety protocol at a Dallas Hospital has caused a female health-care worker to become infected with Ebola after having extensive contact with Ebola patient Thomas Eric Duncan.

The development – the first known case of the deadly disease transmitted in the United States – stunned health officials who had not included the woman in the group of 48 people being monitored for the disease because she was thought to be of low risk for infection because she had worn protective equipment while caring for Duncan.

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First responders guard the apartment of a health care worker Sunday in Dallas. The health care worker, who was caring for Ebola patient Thomas Eric Duncan, tested positive for the disease in preliminary tests. The Associated Press

Centers for Disease Control and Prevention Director Dr. Tom Frieden speaks at a news conference, Sunday Oct. 12, 2014, in Atlanta. Top federal health officials said Sunday that the Ebola diagnosis in a health care worker who treated Thomas Eric Duncan at a Texas hospital clearly indicates a breach in safety protocol. But the unidentified worker has been unable to pinpoint where that breach might have occurred, according to Frieden. (AP Photo/John Amis)

Dr. Tom Frieden, head of the Centers for Disease Control and Prevention, speaks at a news conference, Sunday Oct. 12, 2014, in Atlanta. A Texas health care worker, who was in full protective gear when providing hospital care for Ebola patient Thomas Eric Duncan, who later died, has tested positive for the virus and is in stable condition. (AP Photo/John Amis)

A hazmat worker moves a barrel while finishing up cleaning outside an apartment building of a hospital worker Sunday in Dallas. The Texas health care worker, who was in full protective gear when they provided hospital care for Ebola patient Thomas Eric Duncan, who later died, has tested positive for the virus and is in stable condition, health officials said Sunday. The Associated Press

A worker in safety garb packs up a barrel while cleaning outside an apartment building of a hospital worker Sunday in Dallas. The health care worker, who helped care for Ebola patient Thomas Eric Duncan, has tested positive for the virus and is in stable condition. The Associated Press

A hazmat worker points to the entrance of an apartment building of a hospital worker, Sunday, Oct. 12, 2014, in Dallas. The Texas health care worker, who was in full protective gear when they provided hospital care for Ebola patient Thomas Eric Duncan, who later died, has tested positive for the virus and is in stable condition, health officials said Sunday. (AP Photo/LM Otero)

But the woman developed symptoms on Friday, and preliminary diagnostic tests came back positive late Saturday – a reminder of the risk that nurses, doctors and other hospital workers face treating Ebola patients. More than 400 health-care workers, the vast majority in West Africa, have contracted Ebola. Fifty-eight percent of those have died, according to the most recent report from the World Health Organization.

Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said the CDC still must confirm the diagnosis, but that there was little doubt that there had been a failure of the supposedly rigid precautions that health workers are to take when caring for patients with the contagious disease.

“We don’t know what occurred in the care of (Duncan), but at some point, it was a breach in protocol and that breach in protocol resulted in this infection,” Frieden said in a briefing on Sunday. “If this individual was exposed, which they were, it is possible that other individuals were exposed,” Frieden said.

All health-care workers who treated Duncan are now being monitored for possible infection. Previously, those monitored included family members and others who had come into contact with Duncan before he was hospitalized Sept. 28. Four family members are being checked twice a day and are being guarded to ensure they remain isolated. The 21-day incubation period for most of the 48 ends on Oct. 19.

As the epidemic has swept the West African nations of Liberia, Guinea and Sierra Leone, health-care workers, unaware that they were treating people with the disease, are thought to have played a role in its spread. Texas officials said only one person is believed to have had contact with the new Dallas patient after she began to show symptoms. That person will now be monitored for 21 days.

The CDC now is recommending that the Dallas hospital, Texas Health Presbyterian, keep the number of workers treating possible Ebola patients to an “absolute minimum,” Frieden said. The agency also wants the hospital to provide a full-time infection control officer to ensure that safety measures are followed with Ebola patients.

The hospital announced Sunday that its emergency department had stopped until further notice accepting patients brought by ambulance “because of limitations in staffed capacity,” a step known as “diversion.”

“While we are on diversion we are also using this time to further expand the margin of safety by triple-checking our full compliance with updated CDC guidelines,” the hospital said. “We are also continuing to monitor all staff who had some relation to Mr. Duncan’s care even if they are not assumed to be at significant risk of infection.”

The infected woman sought care immediately after her symptoms developed and was placed in isolation at the hospital, Frieden said.

Frieden said the worker had extensive contact on multiple occasions with Duncan during his care, but currently had a low level of the virus.

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