It is possible more people who came in contact with a man who died from the Ebola virus in Dallas, Texas, could test positive for the virus in the coming days, the head of the Centers for Disease Control and Prevention said Sunday.

Tom Frieden offered this possibility a day after a preliminary diagnosis showed that a health care worker who had “extensive contact” with Thomas Eric Duncan tested positive for Ebola. If the test is confirmed, this would be the first known case of Ebola being contracted or transmitted in the United States.

“We’re very concerned,” Frieden said at a news briefing at CDC headquarters in Atlanta. ” … Our thoughts are with the health care worker.”

The level of the virus in the woman’s system was “low,” Frieden said, adding that another test to confirm whether she has the virus will be conducted later Sunday.

Frieden said at least 48 people who came in contact with Duncan before he was admitted to the hospital in Dallas are at risk of contracting the virus.

“Unfortunately, it is possible in the coming days we will see additional cases of Ebola,” Frieden said.

“The risk is in the 48 people who are being monitored, all of whom have been tested daily, none of whom so far have developed symptoms or fever,” Frieden said.

An “intensive investigation” is also being conducted to determine others who may have come in contact with Duncan while he was being treated, because they may have been exposed as well, Frieden added. Some of these workers could have had a breach in contamination control similar to the one that led to the nurse being infected, he said.

The nurse, who was not identified, worked at Texas Presbyterian Hospital in Dallas and wore protective gear when treating Duncan, who died Wednesday, Frieden said. “But at some point there was a breach in protocol and that breach in protocol resulted in this infection.”

When the nurse became feverish on Friday, she reported it and was given a preliminary test for the virus, which turned up positive Saturday night, Frieden said.

The CDC and Texas health officials are investigating how many people the nurse came in contact with after coming down with symptoms of the disease.

“It appears at this time that there is only one (person) who may have had contact with her,” Frieden said. “That individual is under active monitoring.”

Frieden said the CDC has sent additional staff to Dallas to assist with the response. The agency will also enhance training of health care workers who may have to treat patients with the virus.

“The care of Ebola can be done safely, but it is hard to do it safely,” Frieden said. “It requires meticulous and scrupulous attention to infection control and even a single innocent, inadvertent slip can result in contamination.”

The key to stopping the spread of the disease is to “break the chains of transmission,” he said. This involves promptly diagnosing anyone who has symptoms of the disease, isolating that individual, identifying everyone this person came in contact with and actively monitor those people over a 21-day period. If any of these contacts comes down with Ebola symptoms, then the same process starts over again.

“That’s how we have stopped every Ebola outbreak in history, except the one in West Africa,” Frieden said. “That’s how we stopped it in Lagos, Nigeria. That’s how we will stop it in Dallas.”

David Lakey, commissioner of the Texas Department of State Health Services, who also participated in the news briefing, agreed.

“I firmly believe we will stop it,” Lakey said.

“Our hearts really do go out to this individual and to the family,” he added. “She was a health care worker who was willing to compassionately care for Mr. Duncan. Our thoughts and prayers are with them.”

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