Warnings about the current Ebola epidemic in West Africa, which has sickened more people than all previous outbreaks combined, have been dire for a while now.

In March, a Doctors Without Borders official called it "an epidemic of a magnitude never before seen." By June, the organization was already warning that it was "totally out of control." In early September, the director of the Centers for Disease Control and Prevention said we still had a chance to stop the outbreak, but that the "window of opportunity [was] closing."

Soon after, President Obama called for action in a press conference, saying the epidemic was "spiraling out of control," and the World Bank called the potential economic impacts of the outbreak "catastrophic."

Predicting the future of events like disease outbreaks, with so many variables and moving targets, is difficult. Estimates may be far off base. But projections about the worst that could happen if the international community does not step up its efforts to contain the Ebola epidemic have become increasingly alarming.

A new paper in the New England Journal of Medicine by a team of World Health Organization researchers estimates that if current trends continue, the number of confirmed and probable cases will rise to 20,000 by November 1, with almost half of those cases in Liberia:

Observed (triangles) and projected (dotted line) cases of Ebola in the three hardest-hit countries. WHO Ebola Response Team / NEJM

The Centers for Disease Control and Prevention also released projections on Tuesday, using a new modeling tool that ended up with different numbers than the predictions published in the NEJM.

They estimated that reported cases in Sierra Leone and Liberia could rise to 8,000 by the end of September. But the CDC also noted that cases seem to be vastly underreported, perhaps by a factor of 2.5. According to the new projections, the true number of cases by the end of September could — worst case scenario — rise to 21,000.

By January 20, 2015, the number of Ebola cases in Liberia and Sierra Leone could be as high as 550,000, the CDC report said. Correcting for underreporting, the number would be vastly higher: 1.4 million cases.

"If conditions continue without scale-up of interventions," the CDC researchers warned, "cases will continue to double approximately every 20 days, and the number of cases in West Africa will rapidly reach extraordinary levels."

The projection of over 1 million cases by the end of January is not the likeliest scenario, but it is entirely possible. "My gut feeling is, the actions we're taking now are going to make that worst-case scenario not come to pass," CDC director Tom Frieden told The New York Times. "But it’s important to understand that it could happen."

There are reasons to think that the tide may be turning against Ebola. "The findings... indicate that the epidemic can be controlled," the CDC researchers wrote, and the international community has already begun mobilizing to help avoid the worst-case-scenario predictions. (The CDC's best-case-scenario projections show that the epidemic could be almost over — rather than infecting more than a million people — by January 20.)

Ebola is spread via bodily fluids, and isolating patients and their contacts is the most effective way to contain it. More beds set up for Ebola treatment are desperately needed.

The CDC projections — based on data from August — "reflect a moment in time before recent significant increases in efforts to improve treatment and isolation," said Frieden in a statement. "They do not account for actions taken or planned since August by the United States and the international community. We anticipate that these actions will slow the spread of the epidemic."

This post was updated with additional information from the CDC.