A U.S. health-care worker exposed to the Ebola virus in Sierra Leone arrived at Nebraska Medical Center's Biocontainment Unit Sunday for observation. (Nebraska Medicine)

A U.S. health-care worker exposed to the Ebola virus in Sierra Leone arrived at a Nebraska hospital on Sunday for observation and potential treatment.

The patient, who has not been identified, arrived at the Nebraska Medical Center’s Biocontainment Unit mid-afternoon.

“Even though the patient hasn’t had a positive test for Ebola, all of our team members are taking the same precautions that were taken with the first three patients who did have Ebola,” spokesman Taylor Wilson said. “It’s out of an abundance of caution that this approach is being used.”

The patient will be under observation for the recommended 21-day monitoring period in the same room that was used to treat three patients with Ebola: doctor Richard Sacra, in September; NBC cameraman Ashoka Mukpo, in October; and doctor Martin Salia, who died after less than two days of treatment in November.

The current Ebola epidemic is the largest in history, mostly affecting the West African countries of Guinea, Liberia and Sierra Leone, according to the Centers for Disease Control and Prevention, which has more than 150 staff members on the ground in the region. As of Friday, there have been more than 20,000 cases worldwide, with just over 8,000 total deaths, according to the CDC and the World Health Organization.

Sen. Chris Coons (D-Del.), who recently visited Liberia, where troops have been deployed to assist in the response to the outbreak, said Sunday that the troops there have done “remarkable work,” but he called on the Pentagon to change its strategy.

Although “we can’t declare mission accomplished and withdraw too early here, we can bring home a thousand or more of these troops now,” he said on CBS News’ “Face the Nation.” “They’re currently bored because they’ve accomplished a lot of their mission of building infrastructure.”

What was a “raging epidemic” in September in Liberia is now “down to a few embers scattered across this country,” Coons said. Some troops should remain for the rest of the year to help transition the response locally, he said, adding that he would formally call for the shift in strategy on Monday in notes to the president and the defense secretary.

The Nebraska Medical Center has notified the CDC about the patient but has yet to ask for its involvement. Should the Center request assistance, the CDC’s role would be an advisory one, said CDC spokeswoman Kathy Harben.

“Our role in situations like this is that we consult about infection control during the whole process and then, of course, we’re available to Nebraska Medical Center with any other questions they have,” she said.

Liberian Thomas Eric Duncan was the first Ebola patient diagnosed in America and died in Dallas in October. Two nurses tending to Duncan tested positive for the virus but have since recovered, and another health-care worker, doctor Craig Spencer, was diagnosed with Ebola in October but was later declared “Ebola-free.” Hundreds of individuals in Texas, Ohio and New York who may have come in contact with those patients had been monitored by the CDC.

A team of experts assembled by Berlin’s Robert Koch Institute last week offered one theory on the source of the current epidemic: a 2-year-old boy who may have contracted Ebola from infected bats.

“The severe Ebola virus disease epidemic occurring in West Africa likely stems from a single zoonotic transmission event involving a 2‐year‐old boy in Meliandou, Guinea, who might have been infected by hunting or playing with insectivorous free‐tailed bats living in a nearby hollow tree,” they write in the opening of their report.

That theory is based off of research and interviews with locals, who say children often played near the tree.