Nearly a dozen US aid workers were evacuated from Sierra Leone last week after being exposed to the deadly Ebola virus.

The workers were assisting patients at a Partners in Health clinic in Port Loko district, and one of them tested positive for Ebola virus.

The infected worker is in serious condition, and receiving treatment at the National Institutes of Health (NIH) in Maryland. The other workers have not shown signs of infection, according to the Centers for Disease Control (CDC). They will remain in quarantine for the 21-day incubation period.

The CDC is investigating how the worker became infected, and who else was exposed to the patient. Another American aid worker was flown into the US on March 13 after showing signs of Ebola virus infection, although two tests came back negative.

The worker at NIH is the 11^th person infected with the Ebola virus to be treated in the US. Around 10,000 people have died from the current outbreak, which began in Guinea a year ago. There have been over 24,000 reported cases of infection.

President Obama announced last month that all but 100 American troops sent to West Africa to help manage the Ebola outbreak will be withdrawn by April 30. The troops were dispatched to help contain the deadly virus, but the President said that the mission in the ravaged countries has shifted.

"While our troops are coming home, America's work is not done," he insisted. "Our mission is not complete. Today we move into the next phase."

Obama said that the focus has changed from containment to eradication, and said that civilian aid workers will be increased in the area. Civilian government employees, volunteers, and the remaining troops will help in that effort.

Ebola cases in Liberia have generally decreased, while transmissions in Guinea and Sierra Leone continue to trouble health officials.