Federal health officials are warning travelers to avoid three West African countries where an Ebola outbreak is getting worse, a sign of the U.S. government's growing concern about the virus.
The Centers for Disease Control (CDC) issued a "Level 3" Warning on Thursday, the agency's most serious travel notice, telling people in the United States not to enter Guinea, Liberia and Sierra Leone unless absolutely necessary.
"This is the biggest and most complex Ebola outbreak in history," Frieden said in a statement. "Far too many lives have been lost already. It will take many months, and it won't be easy, but Ebola can be stopped. We know what needs to be done."
The CDC is not screening passengers traveling from the three countries at points of entry, it said, noting that Ebola is not contagious until symptoms appear. The virus has no treatment and a fatality rate of almost 100 percent.
The outbreak is believed to have begun in January, with initial cases reported for the first time in February.
The death toll rose to at least 729 on Thursday, even as additional cases brought the total number infected to more than 1,300, according to the World Health Organization.
Leaders in West Africa are taking extraordinary steps to contain the virus. Notably, Sierra Leone President Ernest Bai Koroma announced that he will use troops to quarantine infected areas. Koroma also canceled a planned trip to the United States, according to The New York Times.