(NBC News) -- The ebola outbreak is getting worse, and the United States is warning travelers to stay away from affected regions.
More than 700 people have died and more than 1,300 others have been infected in West Africa.
"This the toughest ebola outbreak we've ever had to face, and ebola itself is a dreadful and merciless virus," says Centers for Disease Control Director Dr. Tom Frieden.
The CDC is beefing up its own response and staffing in the region, but has issued a warning against all non-essential travel to Guinea, Sierra Leone and Liberia.
On Wednesday night church members in Charlotte, North Carolina held a prayer service for the two infected Americans in Liberia.
Dr. Kent Brantly and Nancy Writebol are from the charity organizations Samaritan's Purse and a group called Serving in Mission.
On Thursday Samaritan's Purse said in a statement that an "experimental serum" was sent in hopes of treating the patients. There was only enough for one person.
Although Dr. Brantly's condition worsened overnight, he asked that the serum be given to Writebol.
Meanwhile, attempts to save Dr. Brantly's life include a blood donation from a teenage boy who recovered from ebola, thanks to Brantly's earlier efforts.
-- Erika Edwards reports