Four Americans who may have been exposed to the deadly Ebola virus while visiting Sierra Leone have arrived in the United States and are under observation at the University of Nebraska Medical Center, according to the hospital.

The individuals are being housed at the University’s campus in Omaha while hospital personnel monitor them for Ebola symptoms. They will be evaluated for 21 days to make sure that none are carrying virus, according to the hospital spokesman Taylor Wilson. “They’re not sick and not contagious,” Wilson assures the U.S. populace.

These four individuals in Nebraska are among at least 10 Americans flown to the U.S. via non-commercial air transport who may have been exposed to the virus. They may have had contact with an unidentified Ebola patient in Sierra Leone, or had similar exposure to the deadly virus. While Ebola has killed about 10,000 people in Sierra Leone, Liberia, and Guinea during its latest epidemic, only a handful of cases have been confirmed in the U.S. – most of which were contracted in West Africa.

Meanwhile, a U.S. healthcare worker who tested positive for Ebola also arrived back in America this weekend. The patient is currently at the National Institutes of Health in Bethesda, Maryland, and in serious condition, according to a statement by the NIH on Friday. Other health workers who came in contact with their colleague will be flown in for observation on Monday, although none currently show any symptoms. The workers are clinicians for Partners in Health, a Boston-based aid group.

Other aid workers with possible Ebola exposure have been scheduled to fly into Atlanta and housed near the Emory University Hospital, according to Nancy Nydam, spokeswoman for the Georgia Department of Public Health. All patients will have to stay quarantined in their housing for 21 days after their date of exposure. “Twice a day, we’ll have visual monitoring either face to face or we’ll Skype with them, or do FaceTime,” said Nydam.

Nebraska, the NIH, and Emory are 3 of only 4 hospitals in the U.S. that have biocontainment units able to deal with a highly infectious disease like Ebola.