Debbie Sacra, wife of Dr. Rick Sacra, an American doctor who contracted the Ebola virus in Africa, smiles as she speaks about her husband's health while addressing members of the media at the University of Massachusetts Medical School Thursday Sept. 4, 2014 in Worcester, Mass. (AP Photo/Stephan Savoia)

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.

In this Oct. 28, 2006, file photo, a mock patient is wheeled in an isolation pod during a drill at the Nebraska biocontainment unit in the Nebraska Medical Center in Omaha, Neb. Dr. Rick Sacra, 51, who was infected with Ebola while working in Liberia is being flown to the Nebraska Medical Center in Omaha where he is expected to arrive sometime Friday. Sacra will begin treatment in the hospital's special isolation unit, believed to be the largest in the U.S. (AP Photo/Nati Harnik)

The team is discussing experimental treatments, including using blood serum from a patient who has recovered from Ebola, Smith said. There are no licensed drugs or vaccines for the disease, but about half a dozen are in development.

"We've been trying to collect as much information on possible treatments as we can," Smith said.

Much attention has focused on the unproven drug ZMapp, which was given to seven patients, two of whom died. But the limited supply is exhausted and its developer says it will take months to make even a modest amount.

From left, Dr. Mark Rupp, an infectious disease specialist; Dr. Phil Smith, medical director, and Dr. Angela Hewlett, associate medical director, hold a news conference at the Truhlsen Eye Institute in Omaha, Neb. on Sept. 4, 2014. Doctors at the Nebraska Medical Center said they're ready to treat a missionary who was infected with Ebola while serving in Liberia. Dr. Rick Sacra is expected to arrive in Omaha on Friday. (AP Photo/The Omaha World-Herald, Matt Miller)

Smith and several other doctors with the unit repeatedly said Sacra's transfer to Omaha posed no threat to the public, noting Ebola is transmitted through close contact with an infected person.

Sacra's wife, Debbie, said Thursday that her husband was in good spirits as he boarded the plane. She said the couple had known there was a risk of Ebola infection when he left for Liberia in August.

"I knew he needed to be with the Liberian people," she said. "He was so concerned about the children that were going to die from malaria without hospitalization and the women who had no place to go to deliver their babies by cesarean section.

Dr. Phil Smith, medical director, gives a news conference in Omaha, Neb. on Sept. 4, 2014. Doctors at the Nebraska Medical Center said they're ready to treat a missionary who was infected with Ebola while serving in Liberia. Dr. Rick Sacra is expected to arrive in Omaha on Friday. (AP Photo/The Omaha World-Herald, Matt Miller)