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At least 15 Americans were exposed to Ebola from a single, infected U.S. healthcare worker and have been brought back to the United States, the Centers for Disease Control and Prevention said Tuesday.

They all work for Partners in Health, a nonprofit group that’s been helping fight the Ebola epidemic in West Africa.

Late last week, one person who was potentially exposed was brought to Atlanta to be near Emory University Hospital for observation.

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Over the weekend, Partners in Health said 10 more of its staffers also might have been exposed to the often deadly virus when they were trying to help their sick colleague — who hasn’t been identified and who’s in critical condition at the National Institutes of Health outside Washington, D.C.

Now, another four have been identified and are being brought back for observation. So far, only one person, the first patient, has tested positive for Ebola symptoms.

It’s not clear how so many people got exposed to the virus.

“We’re still investigating and hopefully we‘ll have an answer to that question,” said CDC spokesman Tom Skinner. “The circumstances around all these exposures is what we are looking at right now. “

Ebola’s infected more than 24,000 people in Guinea, Sierra Leone and Liberia and killed more than 10,000 of them. The patient at NIH is the 11th to be treated in the United States.

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-- Maggie Fox