OMAHA, Neb. (AP) — A doctor who became infected with Ebola while working in Liberia is sick, but in stable condition at the Nebraska Medical Center, officials said Friday.

Dr. Rick Sacra, 51, is being treated at the largest of the United States’ four special isolation units. It was built to handle patients with highly infectious and deadly diseases, according to Dr. Mark Rupp, chief of the infectious diseases division at the center.

Sacra— the third American aid worker sickened with the virus — arrived after 6 a.m. Friday at the Omaha hospital for treatment. Rupp said Sacra was wheeled on a gurney off the plane at Offutt Air Force Base, transferred to an ambulance and then wheeled into the hospital. 

The doctor from suburban Boston spent 15 years working at the Liberia hospital where he fell ill, and felt compelled to return after hearing that two other missionaries were sick. Sacra delivered babies at the hospital, and was not involved in the treatment of Ebola patients, so it’s unclear how he became infected with the virus that has killed about 1,900 people.

The first two American aid workers infected by Ebola — Dr. Kent Brantly and Nancy Writebol — have recovered since being flown to Emory University Hospital in Atlanta for treatment.

Dr. Phil Smith, medical director of the Omaha unit, said a team of 35 doctors, nurses and other medical staffers will provide Sacra with basic care, including ensuring he is hydrated and keeping his vital signs stable.