The West African Ebola outbreak has already infected more than 3,000 people and killed more than 1,500 of them, but the World Health Organization said the virus could infect more than 20,000 people before it's brought under control.

In a report aimed at creating a play to end the outbreak, the WHO said containment could take six to nine months and cost up to $490 million, according to USA Today.

"20,000 is a scale that has never been anticipated," said Bruce Aylward, an assistant director general of WHO. "That's not saying we expect 20,000 cases. That's not saying we accept 20,000."

The latest numbers from the WHO show that 3,069 cases of Ebola have been reported in Guinea, Liberia, Nigeria and Sierra Leone, resulting in 1,552 deaths. The overall death rate is 51 percent.

The WHO report said that the number of cases could be two-to-four times higher than reported in heavily affected areas.

Africa has seen Ebola outbreaks before, but they have been contained before the number of infected reached into the thousands. The current outbreak is the largest in recorded history.

In its "road map" to Ebola containment, Aylward said an effective response requires 750 international health workers and 12,000 workers from the affected countries. Right now, he says, a few areas have adequate staffing and facilities but "in other areas we have 10 percent of what we need. In some areas we have zero."

Doctors Without Borders, a humanitarian group working to help contain the virus, said the international response has been "chaotic and inadequate." The organization said it welcomed the WHO plan, but immediate action was the most important thing, Fox News reported.

"Huge questions remain about who will implement the elements in the plan," said Brice de le Vingne, operations director of Doctors Without Borders. "None of the organizations in the most-affected countries ... currently have the right set-up to respond on the scale necessary to make a serious impact."