The director of the Centers for Disease Control and Prevention said a "breach of protocol" caused a health care worker at a Dallas hospital to contract Ebola.

The health care worker, a female nurse at Texas Health Presbyterian Hospital, had "extensive contact" on multiple occasions with Thomas Eric Duncan, the first person diagnosed with Ebola in the U.S., who died last week, Dr. Tom Frieden said in a press conference Sunday. 

The health care worker's case is the first known case of the disease being contracted or transmitted in the U.S.  Frieden said the CDC is evaluating other health care workers who may have had the same breach of protocol as the nurse. 

Dallas County Judge Clay Jenkins said at a press conference Sunday morning that city officials expected it was possible that a second person could contract the virus. 

"Contingency plans were put into place," he said. 

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Jenkins said he wanted to stress Ebola cannot be contracted unless one comes into contact with the bodily fluids of an Ebola victim. 

"You cannot contract it by walking by people on the streets," he said. "There is nothing about this case that changes that basic premise of science."

Dr. Daniel Varga, of the Texas Health Resource, said the worker was in full protective gear when they provided care to Duncan during his second visit to the hospital. 

Varga said the family of the worker has "requested total privacy."

Varga said the health care worker reported a fever Friday night as part of a self-monitoring regimen required by the CDC.

Dallas Mayor Mike Rawlings said the Dallas Fire Department's rescue hazmat team has decontaminated any open areas of the health care worker's apartment complex. 

"Police are standing by to make sure no one enters that apartment complex," he said.

Rawlings said officials have knocked on every door within a block of the apartment and have spoken with every person that came to the door. Reverse 911 calls have been made to residents within four blocks of the apartment complex and printed materials have been left at each door, he said. 

A team has decontaminated and secured the vehicle the health care worker drove to the hospital. Rawlings said hazmat units will go into the worker's apartment and clean up the interior Sunday. 

"We had this plan in place last week, so when we got this phone call, which we thought we might get, we put an action team in place," Rawlings said.

"We knew a second case could be a reality, and we've been preparing for this possibility," said Dr. David Lakey, commissioner of the Texas Department of State Health Services. "We are broadening our team in Dallas and working with extreme diligence to prevent further spread."

Health officials have interviewed the patient and are identifying any contacts or potential exposures. They said people who had contact with the health care worker after symptoms emerged will be monitored based on the nature of their interactions and the potential they were exposed to the virus.

Ebola spreads through close contact with a symptomatic person's bodily fluids, such as blood, sweat, vomit, feces, urine, saliva or semen. Those fluids must have an entry point, like a cut or scrape, or someone touching the nose, mouth or eyes with contaminated hands, or being splashed. The World Health Organization says blood, feces and vomit are the most infectious fluids, while the virus is found in saliva mostly once patients are severely ill. The whole live virus has never been culled from sweat, WHO says.

Duncan, the first person diagnosed with Ebola in the U.S., died Wednesday in Dallas. Duncan, 42, grew up next to a leper colony in Liberia and fled years of war before later returning to his country to find it ravaged by the disease that ultimately took his life.

Duncan arrived in Dallas in late September, realizing a long-held ambition to join relatives. He came to attend the high-school graduation of his son, who was born in a refugee camp in Ivory Coast and was brought to the U.S. as a toddler when the boy's mother successfully applied for resettlement.

The trip was the culmination of decades of effort, friends and family members said. But when Duncan arrived in Dallas, though he showed no symptoms, he had already been exposed to Ebola. His neighbors in Liberia believe Duncan become infected when he helped a pregnant neighbor who later died from it. It was unclear if he knew about her diagnosis before traveling.

Duncan had arrived at a friend's Dallas apartment on Sept. 20 -- less than a week after helping his sick neighbor. For the nine days before he was taken to a hospital in an ambulance, Duncan shared the apartment with several people.

The Associated Press contributed to this report.