ATLANTA -- The Ebola virus has killed more than 700 people in Africa and could have catastrophic consequences if allowed to spread, world health officials say. So why would anyone allow infected Americans to come to the U.S.?

The answer, experts say, is because the hospital in Atlanta where they will be treated -- Emory University Hospital -- is one of the safest places in the world to treat someone with Ebola. There's virtually no chance the virus can spread from the hospital's super-secure isolation unit.

And another thing, they say: U.S. medical workers risking their lives overseas deserve the best treatment they can get.

Dr. Kent Brantly became the first person infected with Ebola to be brought to the United States from Africa, arriving Saturday. Fellow aid worker Nancy Writebol was expected to arrive in several days

Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, told NBC on Sunday that Brantly's condition seems to have improved and that it was encouraging to see Brantly walk out of the ambulance unassisted when he arrived at the hospital.

Frieden said he understands the public's concerns about Ebola, and the public health role is to ensure that the infection is not spread.

"Ebola is very deadly. And it's normal to be scared of deadly diseases," he said.

Emory's infectious diseases' unit was created 12 years ago to handle doctors who get sick at the CDC. It is one of about four in the country equipped with everything necessary to test, treat and contain people exposed to very dangerous viruses.

In 2005, it handled patients with SARS, which unlike Ebola can spread when an infected person coughs or sneezes.

In fact, the nature of Ebola -- which is spread by close contact with bodily fluids and blood -- means that any modern hospital using standard, rigorous, infection-control measures should be able to handle it.

Still, Emory won't be taking any chances.

Inside the unit, patients are sealed off from anyone who doesn't wear protective gear.

"Negative air pressure" means air flows in, but can't escape until filters scrub any germs from patients. All laboratory testing is conducted within the unit, and workers are highly trained in infection control. Glass walls enable staff outside to safely observe patients, and there's a vestibule where workers suit up before entering. Any gear is safely disposed of or decontaminated.

Family members will be kept at a distance for now, the doctors said. The unit "has a plate glass window and communication system, so they'll be as close as 1-2 inches from each other," said Dr. Bruce Ribner, who will be treating the patients.

Dr. Jay Varkey, an infectious disease specialist who will be treating Brantly and Writebol, gave no word Saturday about their condition. Both have been described as critically ill after treating Ebola patients at a missionary hospital in Liberia, one of four West African countries hit by the largest outbreak of the virus in history.

There is no cure for the virus, which causes hemorrhagic fever that kills as many as 60 per cent to 80 per cent of the people it infects in Africa. There are experimental treatments, but the missionary hospital had only enough for one person, and Brantly insisted that Writebol receive it. His best hope in Africa was a transfusion of blood including antibodies from one of his patients, a 14-year-old boy who survived thanks to the doctor.

There was also only room on the plane for one patient at a time. Writebol will be next, following the same route to Emory in several days.