Brantly was transported from Liberia, where he was treating Ebola patients with aid group Samaritan's Purse, to Emory University Hospital in Atlanta on Saturday.
The hospital is situated near the headquarters for the CDC, which has been tasked with making sure the deadly virus doesn't spread.
"Well, it's encouraging that he seems to be improving," Frieden said during an appearance on CBS' Face the Nation. "That's really important. And we're hoping he'll continue to improve. But Ebola is such a scary disease because it's so deadly. The plain fact is we can stop it, we can stop it from spreading in hospitals and we can stop it in Africa where it is really the source of the epidemic and where we're surging our response so that we can control it there."
Samaritan's Purse, the organization with which another aid worker diagnosed with Ebola worked, Nancy Writebol, issued a statement saying Brantly received an experimental serum to treat Ebola before leaving Liberia.
"We praise God for the news that Kent's condition is improving," the statement said.
Writebol is expected to be transported to Atlanta within the next few days, the aid group said.
"We thank God that they are alive and now have access to the best care in the world," said Franklin Graham, president of Samaritan's Purse. "We are extremely thankful for the help we have received from the State Department, the CDC, the National Institute of Health, WHO and, of course, Emory Hospital."
The symptoms of the Ebola virus, also known as Ebola hemorrhagic fever, include fever, headache, joint and muscle aches, weakness, diarrhea, vomiting, stomach pain, lack of appetite, and bleeding inside and outside the body, the CDC said.
The World Health Organization estimates some 1,323 people in Guinea, Liberia, Nigeria and Sierra Leone have contracted the virus with 729 suspected deaths.
There have been 909 confirmed cases with 485 known deaths.