A fourth patient infected with Ebola virus has arrived in the United States for treatment. The World Health Organization doctor arrived at Emory University Hospital in Atlanta and hasn’t been named — a WHO spokesman says the agency wants to preserve the patient’s privacy. He shuffled into the hospital from the ambulance that carried him from a nearby air force base. Emory’s where the first two U.S. patients — Nancy Writebol and Dr. Kent Brantly — got treated. They were released and are healthy. Dr. Rick Sacra is being treated at the University of Nebraska Medical Center.

"We were confident of our ability to safely take care of the first two patients when they arrived," Emory's Dr. Aneesh Mehta told reporters. "We have the same level of confidence and security this time." Both Emory and Nebraska have special biocontainment isolation units. Experts say they are not needed to treat Ebola patients — good isolation practices should keep patients, workers and the public safe — but the extra layer adds assurances for a worried public. Writebol, Brantly and Capra have all received experimental therapies, although Mehta has said good supportive care including careful balancing of electrolytes such as potassium and magnesium, as well as good nutrition, probably saved their lives.

Sacra is improving, his doctors say. "He’s kind of becoming his normal self,” said Dr. Angela Hewlett, associate medical director of the biocontainment unit in Nebraska. “Family members continue to speak with Dr. Sacra on a regular basis via video conference and that’s a big help for both the patient and his family.” WHO says more than 4,200 people have been infected in this epidemic and more than 2,200 have died.

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— Maggie Fox