The fourth American aid worker sickened with the Ebola virus arrived Tuesday morning to a mostly calm scene at Emory University Hospital, where two others had been successfully treated.

An ambulance carrying the patient arrived about 10:25 a.m., with a police escort. Wearing a bulky protective suit similar to those of Emory's first two arrivals, the patient walked from the ambulance to the hospital.

About an hour earlier, the specially equipped plane carrying the patient touched down at Dobbins Air Reserve Base, located northwest of Atlanta.

"Just as we did in three previous cases, every precaution was taken to move the patient safely and securely, to provide critical care en route, and to maintain strict isolation upon arrival in the United States," said Marie Harf, State Department Deputy Spokesperson.

Emory University Hospital said the patient, whose identity and status remain confidential, will be treated in the isolation unit previously used by the other two patients. The World Health Organization (WHO) would only confirm that a doctor who had been working in an Ebola treatment center in Sierra Leone tested positive for the disease. The State Department later confirmed the doctor was from the U.S.

At a news conference, Dr. Aneesh Mehta said the patient's ability to walk from the ambulance was a good sign, but not the only one the hospital must consider.

"We will evaluate all options, and with the patient make a decision about what avenues we explore," Mehta, an infections disease expert at Emory, said. "We have protocols in place.”

The special isolation unit that will house the patient can hold up to three people, and that capacity could be increased if needed, Mehta said.

Last month, two U.S. aid workers who contracted Ebola in Liberia, Dr. Kent Brantly and Nancy Writebol, were treated successfully at Emory.

"We were comfortable taking care of these patients," Mehta said. "We understand that there was fear out there but we hope that in our ability to communicate our processes and educate the public that that fear is being diminished.”

Another aid worker, Dr. Rick Sacra, 51, of Worcester, Massachusetts, is being treated at the Nebraska Medical Center in Omaha. He arrived Friday and is in stable condition. Sacra was practicing family medicine in Liberia with the North Carolina-based charity SIM, with whom Writebol also worked.

"He hasn't be able to eat much since he got here, but he had some toast and applesauce," Debbie Sacra, his wife, said Tuesday. "He also tolerated the research drug well -- better than he had the previous doses he was given.”

Federal officials say they asked the Nebraska hospital to treat him instead of Emory in order to prepare other isolation units for more Ebola patients if needed.

The Ebola outbreak sweeping West Africa has killed more than 2,200 people and has taken a particularly high toll on health care workers. More than 4,200 people have believed to have been sickened in Guinea, Liberia, Sierra Leone, Nigeria and Senegal.

Dr. Daniel Bausch, a tropical disease expert and associate professor at Tulane University said the outbreak is causing a "tense, difficult situation in West Africa."

"There are not enough health workers," Bausch said, adding that the Ebola center where the latest American patient contracted the disease is "struggling to get the amount of staff it needs."

"There is not the degree of safety we'd like," Bausch said. In an effort to address these concerns, he is working with WHO officials to conduct a clinic that will train health care workers headed into Ebola-infected regions.

The WHO has suggested turning to the blood of Ebola survivors as an experimental treatment, and Sacra's doctors have said they are considering it.  

Mehta said Emory doctors have been advising other physicians that some particular types of supportive care did seem to help. Those included switching between different types of IV fluids to meet each patient's specific electrolyte needs at the time. Mehta said medical staff was also giving high-quality liquid nutrition to boost levels or protein and other nutrients "to help build back that immune system that was under attack."

Both Brantly and Writebol were given the experimental drug ZMapp, and credit the drug with helping their recovery, though there is no way to know its effects. Sacra is being treated with a different experimental drug that his doctors have refused to name, but say they've been consulting with Ebola experts.

Details of the latest patient's treatment are not known, but ZMapp could not have been used. Brantly and Writebol were the first to receive it; it had never been tested on humans. The rest of the limited supply was given to five others.

Dr. Gary Kobinger of the Public Health Agency of Canada helped pioneer the research that led to ZMapp and said the U.S. manufacturer appears to be on track for a Phase 1 safety study early next year.

Pharmaceutical companies are working to develop vaccines for Ebola and other alternative drugs to treat the disease, but they're not fully tested or readily available yet. Kobinger said a Canadian-made vaccination candidate should be starting Phase 1 trials within weeks.

At Emory on Tuesday, law student Grace Van Dyke said she had heard that some people around the country were initially concerned about Ebola patients being brought to the U.S. But she never heard worries from the university community.

"Those of us who are at Emory, we're not concerned because we know the quality of Emory medical care, and we know the reason they were brought here is because Emory is capable of containing it and treating them," she said.

Fox News' John Roberts and The Associated Press contributed to this report.