OMAHA, Neb. — A doctor who became infected with Ebola while working in Liberia — the third American aid worker sickened with the virus — is sick but in stable condition and communicating with his caregivers at the Nebraska Medical Center.

Dr. Rick Sacra, 51, is being treated at a 10-bed special isolation unit, the largest of the four in the U.S., officials said Friday. 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 arrived at 6:38 a.m. Friday at the Omaha hospital. He was wheeled on a gurney off the plane at Offutt Air Force Base, transferred to an ambulance and then wheeled into the hospital, said Rosanna Morris, chief nursing officer for the medical center.

Sacra was conscious Friday and was able to communicate with medical staff, Morris said.

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. Sacra came to Omaha instead of Atlanta because federal officials asked the medical center to treat him to prepare other isolation units to take more Ebola patients if needed.

Sacra, a doctor from Worcester, Mass., who spent 15 years working at the Liberia hospital where he fell ill, 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.