The largest-ever outbreak of Ebola could eventually affect 20,000 people—seven times more than the current number of cases—before it runs its course, according to a new report from the World Health Organization. Months after the disease was first identified, reported cases are still sharply rising.

WHO numbers released today bring the death count up by more than 100 to 1,552, the New York Times reports, and the number of cases to 3,069. Forty percent of those cases occurred within the last 21 days.

Three countries—Sierra Leone, Guinea, and Liberia—continue to bear the brunt of the outbreak, though Nigeria, Africa's most populous nation, has recorded 15 Ebola patients, six of whom have died. The disease made its way to the country after Patrick Sawyer, a Liberian-American man infected with Ebola, flew to Lagos, the nation's capitol, and died.

The WHO report acknowledges that the number of current cases may be two to four times higher than reported, and that the epidemic "continues to accelerate."

"The cases are increasing. I wish I did not have to say this, but it is going to get worse before it gets better," Tom Frieden, director of the Centers for Disease Control and Prevention, said at a press conference yesterday. "The world has never seen an outbreak of Ebola like this. Consequently, not only are the numbers large, but we know there are many more cases than has been diagnosed and reported."

[Image via AP]