A third American doctor has become infected with the Ebola virus while working in Liberia. Dr. Rick Sacra, 51, has been working in Liberia since 1995. He returned to the country on Aug. 1 after traveling back and forth from Holden, Massachusetts, where his wife and three sons reside. Sacra works for SIM, a missionary group with whom two other Americans who have contracted Ebola, including fellow doctor Kent Brantley, are affiliated. Sacra was not directly treating Ebola patients but delivering babies in the obstetrics unit. “He signed up to serve God by being a missionary doctor,” said Sacra’s brother. “He’s been taking care of people all his life. That’s what he does.” It is unclear whether Sacra will return to the U.S. for treatment. On Tuesday, CDC Director Tom Frieden warned the “window is closing” to stop Ebola’s spread. Thus far, 3,000 people have contracted Ebola during this latest outbreak, killing more than 1,800.