The death rate in the Ebola outbreak has risen to 70 percent and there could be up to 10,000 new cases a week in two months, the World Health Organization warned Tuesday.

Streets in the capital of Sierra Leone were deserted on Friday as the West African state began a contested, three-day lockdown in a bid to halt the worst Ebola outbreak on record.

Although few medical experts realize it, part of the population in West Africa is immune to the Ebola virus, according to virologists who specialize in the disease.

If you'd asked public-health experts a year ago whether an Ebola outbreak could turn into an epidemic spread across borders, they probably would have confidently told you that there was no way: the virus isn't transmitted very easily, and people usually get so sick and die so quickly, it has little opportunity to infect a new host.

"It's even worse than I'd feared," Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, said Wednesday of the Ebola outbreak rampaging through West Africa. "Every day this outbreak goes on, it increases the risk for another export to another country.