Kent Brantly, the Ebola-stricken American doctor who was flown back to the United States from Liberia on Saturday, "seems to be improving," Tom Frieden, head of the Centers for Disease Control and Prevention said Sunday.

"It's encouraging that he seems to be improving," Frieden told CBS' "Face the Nation." "And we're hoping he'll continue to improve. But Ebola is such a scary disease because it's so deadly. I can't predict the future for individual patients."

Brantly, 33, is being treated in a special isolation unit at Emory University Hospital in Atlanta, one of four facilities in the country equipped to deal with infectious diseases. He is believed to be the first Ebola patient ever to be treated on U.S. soil.

Nancy Writebol, a 59-year-old American who was also infected with Ebola while working as a medical missionary in Liberia, is expected to arrive early this week — raising concern that the disease might spread in the United States.

But the CDC says that such concerns are unwarranted, because Ebola can only be spread through direct contact with bodily fluids or blood, and patients exhibiting symptoms are immediately placed in isolation.

"I hope that our understandable fear of the unfamiliar does not trump our compassion, when ill Americans return to the U.S. for care," Frieden said on "Face the Nation."

Frieden said his office has received "nasty emails" and at least 100 telephone calls from Americans concerned that Brantly and Writebol would be allowed to return to the Unted States.

"The plain truth is that we can stop Ebola," Frieden said on ABC's "This Week With George Stephanopoulos." "We know how to control it."

Samaritan’s Purse, one of two faith-based groups sponsoring the missionaries, said Brantly received a dose of an experimental serum before leaving Liberia.

On Sunday, Brantly's wife said she was grateful to have him back in the United States.

“Our family is rejoicing over Kent's safe arrival, and we are confident that he is receiving the very best care," Amber Brantly said in a statement. "We are very grateful to the staff at Emory University Hospital, who have been so nice and welcoming to us. I was able to see Kent today. He is in good spirits. He thanked everyone for their prayers and asked for continued prayer for Nancy Writebol's safe return and full recovery.”

Story continues

Ebola has resulted in the deaths of more than 700 people in Liberia, Guinea, Sierra Leone and Nigeria since February, according to the World Health Organization, and has sickened more than 1,300 in what health officials say is the largest outbreak ever of the hemorrhagic virus.

Frieden said that while the United States may see a few cases of Ebola from people coming into the country from overseas, it is unlikely the virus will spread.

"The single most important thing we can do to protect Americans is to stop this disease at the source in Africa," he said on ABC. "The tried and true public health mechanisms work. We find the patients. You isolate them. You find out who their contacts were. You trace the contacts. You track them every day for 21 days. If they get fever, you start that process again. You make sure there's good infection control, and you educate the community in Africa about safe burial practices. When you do those simple things, Ebola stops.

"Ebola is really like a forest fire," he added. "If you leave one ember burning, it can flare up again."