The U.N. health agency says the death toll from the Ebola virus has more than doubled to 31 in northeastern Congo in recent days.

The World Health Organization says there have been 69 cases now, including nine confirmed by a lab. Five of the deaths have been confirmed by lab work.

WHO spokeswoman Fadela Chaib told reporters Friday in Geneva that "the situation is serious."

Adding to the mystery, she said, is that "it is unusual that the first person to be infected with Ebola was a health worker" and not someone working outside.

Authorities said last week that the outbreak had killed 15 people in northeastern Congo.

Ebola has no cure and is deadly in 40 percent to 90 percent of cases. The disease causes severe internal bleeding.