At least 10 Americans who might have come into contact with the deadly Ebola virus in Sierra Leone are being flown back to the United States for observation, officials said on Saturday. The transfer comes amid concerns the citizens could have been exposed to an unidentified US health worker who recently tested positive for the disease while volunteering in West Africa. The worker was flown in a private jet to a National Institutes of Health (NIH) hospital on Friday. They are said to be in a serious condition. It is not clear how they became infected with Ebola, . The further 10 or more people who are being evacuated to the US are free of Ebola symptoms, said the Centers for Disease Control and Prevention. They will be returned on a non-commercial plane. Scroll down for video . Concerns: At least 10 Americans who might have come into contact with the deadly Ebola virus in Sierra Leone are being flown back to the US for observation. Above, health workers in the West African country . Treatment: The transfer comes amid concerns the citizens could have been exposed to an infected US health worker, who is now in a serious condition at the National Institutes of Health hospital (pictured) in Maryland . Upon their arrival, the Americans will be housed near the University of Nebraska Medical Center in Omaha, the NIH in Bethesda, Maryland, or Emory University Hospital in Atlanta, the CDC said. They will follow the center's  monitoring and movement guidelines during the virus's 21-day incubation period. If someone shows symptoms, they will be sent to an Ebola treatment center for evaluation. More than 10,000 people have died from Ebola - mainly in Sierra Leone, Liberia and Guinea - since December 2013, the World Health Organization says. In total, 24,000 cases have been reported. However, only a handful of these cases have been seen in the United States, Spain and Britain. On Saturday, Thomas Skinner, of the CDC, said an investigation into the infected health worker - and the possible exposure of others - was ongoing, adding more Americans may be evacuated. The NIH, where the unidentified patient is being treated, is one of just four hospitals in the US to have a biocontainment units. It also has infectious disease specialists to care for Ebola patients. Outbreak: More than 10,000 people have died from Ebola, mainly in Sierra Leone, Liberia and Guinea, since December 2013, the World Health Organization says. Above, the number of cases and deaths in thousands . Infected: This graph shows the number of healthcare workers to have been struck down by the deadly virus . Hospital: Upon their arrival, the 10 or so Americans will be housed near the University of Nebraska Medical Center (pictured) in Omaha, the NIH in  Maryland, or Emory University Hospital in Atlanta, the CDC said . The same facility treated US nurse Nina Pham, who was infected with Ebola while caring for Liberian patient, Thomas Eric Duncan, at a Texas hospital. Mr Duncan later died from the virus. Ms Pham, 26, and another nurse, Amber Vinson, who was also infected have since recovered. On Friday, the CDC sent a team to Sierra Leone to investigate how the latest healthcare worker became exposed to Ebola, and determine who may have been in contact with the infected person. CDC spokesman Benjamin Haynes said the center was working out a plan with the U.S. State Department to determine who was coming back and where they will be sent on their return. Well-equipped: The CDC said that one patient was being sent to Emory University Hospital's (pictured) special isolation unit, where several patients have already been treated for the life-threatening disease . Former patient: The NIH, where in the infected health worker is being treated, also cared for US nurse Nina Pham (pictured), who was infected with Ebola while caring for a Liberian patient. She has since recovered . The CDC said that one patient was being sent to Emory University Hospital's special isolation unit, where several Ebola patients have already been treated. Four others are being sent to Nebraska Medical Center to be near their special isolation unit in case they develop Ebola symptoms. Ebola is spread through close contact with the bodily fluids of an infected person. The virus can cause lethal bleeding, muscle aches, severe vomiting and diarrhea. The disease typically takes hold two to 10 days after exposure, but the full inoculation period is considered to last 21 days.
At least 10 American citizens are being flown back to US, CDC has said . Will be evacuated on non-commercial plane and monitored for 21 days . Comes amid concerns they were exposed to Ebola-stricken aid worker . Unnamed American worker remains in NIH hospital in serious condition . Citizens will be housed near  hospitals in Omaha, Maryland and Atlanta . CDC said investigation is ongoing; more Americans might be evacuated . Ebola disease has killed 10,000 people since December 2013, WHO says .