Two American aid workers have been flown back to the United States to be monitored for the Ebola virus after spending time in Sierra Leone.

That brings the number of Americans who have been flown in from the Ebola-stricken West African nation since Friday to 17, although none of them have yet tested positive for the disease, according to an Associated Press report.

All of the 17 individuals were connected to an American, who is as yet unidentified, who returned to the U.S. last week after becoming infected with Ebola. He is in a government hospital in Maryland, where he is listed in critical condition, according to the report.

The other workers will be staying in special isolation units in Maryland, Georgia, and Nebraska to be monitored in case they get sick. The incubation period for Ebola is 21 days, which is how long those suspected of being exposed to Ebola are isolated to make sure symptoms don’t crop up.

A Centers for Disease Control and Prevention (CDC) spokeswoman said that the two who were recently brought back are considered to be a very low risk, but they will be monitored just in case.

The Ebola epidemic that struck the West African nations of Guinea, Sierra Leone, and Liberia last year has only just recently started to slow down after infecting more than 20,000 and killing more than 10,000. The epidemic is the most widely spread case of the Ebola virus in history. It has caused fatality rates of up to 70 percent, and even better than 50 percent among hospitalized patients.

Although there is a 21-day incubation period, symptoms usually take between 4 and 10 days to develop, and begin with symptoms that resemble that of the flu, such as a high fever, fatigue, muscle pain, headache, and sore throat. Later, the patient develops diarrhea, abdominal pain, and vomiting, following by shortness of breath and chest pain and then swelling and confusion.