Margaret Chan

The president warned that the U.S. alone could not halt the epidemic and that without more help to stop the disease, hundreds of thousands of people could die.

Fear of the virus has raised worries about travel to Africa — for safaris in Tanzania, for school programs in South Africa. But London is closer to the outbreak's epicenter than Johannesburg is.

Nearly 2,000 cases have been reported, but staff in West Africa say there are likely many more. The World Health Organization is also coordinating a "massive scaling up" of the international response.

Ebola is spreading faster than efforts to stop it in West Africa, the World Health Organization says. Places hardest hit don't have enough doctors, nurses and medical equipment to contain the virus.