REPUBLIKA.CO.ID, WASHINGTON -- Four Americans who may have beenexposed to the deadly Ebola virus in Sierra Leone have arrived in the United States and are being observed at the University of Nebraska Medical Center, the hospital said on Sunday.

The individuals arrived on Saturday and are being housed on the University of Nebraska Medical Center campus in Omaha, where medical personnel will monitor them for 21 days for any Ebola symptoms, hospital spokesman Taylor Wilson said.

"They're not sick and not contagious," Wilson said.

A U.S. healthcare worker who tested positive for Ebola while in Sierra Leone arrived at the National Institutes of Health in Maryland on Friday and was in serious condition, the NIH said.