Natalie DiBlasio and John Bacon

USA TODAY

A second American stricken with the Ebola virus is expected to return to the USA Tuesday, three days after a doctor being treated for the disease arrived at an Atlanta hospital for intensive care.

Nancy Writebol, a medical missionary aiding in the treatment of Ebola victims in Liberia, is scheduled to leave that African nation around 1 a.m. Tuesday on a specially equipped medical evacuation plane, Liberian Information Minister Lewis Brown said, according to the Associated Press.

Kent Brantly, the American doctor transported to Atlanta Saturday, "seems to be improving," the head of the Centers for Disease Control and Prevention said Sunday.

Brantly is being treated in a special isolation unit at Emory University Hospital in Atlanta after arriving Saturday at Dobbins Air Reserve Base in Georgia. It's the first time anyone infected with the deadly virus has been brought into the country.

"We're hoping he'll continue to improve," Dr. Tom Frieden told CBS' Face the Nation. "But Ebola is such a scary disease because it's so deadly. I can't predict the future for individual patients."

Brantly and Writebol were serving in Liberia as medical missionaries when they became infected with the virus, which has killed 729 people and sickened more than 1,300 the West African nations of Liberia, Guinea and Sierra Leone.

Frieden has said his agency received "nasty" e-mails and at least 100 calls from people questioning why the sick aid workers should be let into the USA. Physicians are confident, however, that the two Americans can be treated without putting the public in danger.

The virus is spread through direct contact with blood, urine, saliva and other bodily fluids from an infected person. It is not spread through the air.

"I don't think it's in the cards that we would have widespread Ebola," Frieden said. He said the virus spreads in African hospitals where there isn't infection control and in burial rituals where people touch the bodies of Ebola victims. That won't happen here, he said.

"So it's not going to spread widely in the U.S. Could we have another person here, could we have a case or two? Not impossible," Frieden said. "We say in medicine never say never. But we know how to stop it here. But to really protect ourselves, the single most important thing we can do is stop it at the source in Africa. That's going to protect them and protect us."

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 severe acute respiratory syndromeSARS, 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.

On Saturday, Amber Brantly expressed her happiness in having her husband back in the USA.

"It was a relief to welcome Kent home today," she said in a statement. "I spoke with him, and he is glad to be back in the U.S. I am thankful to God for his safe transport and for giving him the strength to walk into the hospital."

Brantly, 33, of Fort Worth, had been working in Liberia for Samaritan's Purse overseeing an Ebola treatment center. Writebol, of Charlotte, was working at the center on behalf of the faith group Service in Mission. Samaritan's Purse is paying for their evacuation and medical care.

There's no specific treatment for Ebola so doctors try to ease symptoms, including fever, headache, vomiting and diarrhea. Some victims suffer severe bleeding.

Michael Stulman, a regional information officer in West and Central Africa for Catholic Relief Services, said Sunday that doctors and nurses there "are working in a particularly high-risk environment. They are typically are under-resourced. They're working 15- and even 20-hour days. Despite all their incredible efforts, sometimes mistakes happen and this is why we're seeing a fairly high number of Ebola-positive cases with nurses."

Treatment centers "are reaching full capacity soon after they are set up," he said in an e-mail. "Some of the observation units are not fully staffed. The cross-border nature of the outbreak and the accessibility of transport to major towns have made this outbreak particularly difficult to contain."

Among the main challenges in containing the outbreak, he said, were several circulating myths, including one that said doctors at hospitals "will give you injections that kill you" instead of treating the illness. "As a result of these myths, some families who have sought care are discharging confirmed patients from the hospital and bringing them home before they are recovered. Other people who have Ebola never go to the hospital and instead choose to stay at home with their family, which puts their family at risk of contracting the disease when caring for them."

Contributing: WXIA-TV in Atlanta; Doug Stanglin; the Associated Press