A top Liberian doctor has died and two Americans have become infected in West Africa's Ebola epidemic, which has now spread to the region's most populous city, Lagos, Nigeria.

Until this epidemic, the virus, which kills up to 90 percent of those who fall ill, had struck mainly in small, rural villages. But this outbreak has covered a much broader area—and has killed more than 670 people since late last year. It has now spread to Lagos, and reports came in over the weekend that it had killed a Liberian doctor and sickened two American medical workers, who fell ill after treating patients in Liberia. Both workers are still being treated, and it's not yet clear whether they'll survive.

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