Two scenarios, two completely different outcomes. One has as many as 1.4 million Ebola cases by the end of January, while the other has the epidemic under control and almost over by that time.

Actually, the 1.4 million projection is already out of date, the Centers for Disease Control and Prevention says. It was based on how the Ebola epidemic was going in West Africa in August -- before the U.S. said it was literally sending in the marines, and before other countries said they were going to do more. But the estimate was also calculated before the case count badly worsened.

The action that's already under way is beginning to reverse the worst trends, and a rapid scale-up of help can turn things around, the CDC says.

"A surge now can break the back of the epidemic," CDC director Dr. Tom Frieden told reporters.

The CDC's latest calculations show that at least 8,000 people will have become infected by the end of this month — and maybe as many as 21,000, because it’s clear that most cases are being missed.

That means a lot of deaths — the World Health Organization has revised its death rate estimate to 70 percent of cases, much more than the 50 percent mortality rate now being reported. WHO reports 5,843 people are sick and says 2,803 have died in the epidemic.

“It is still possible to reverse the epidemic, and we believe this can be done if a sufficient number of all patients are effectively isolated, either in Ebola Treatment Units or in other settings, such as community-based or home care,” Frieden added in a statement.

“Once a sufficient number of Ebola patients are isolated, cases will decline very rapidly — almost as rapidly as they rose.”

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CDC researchers made several different calculations about what the Ebola epidemic in West Africa will do. It’s already by far the worst outbreak ever seen, and the first to rise to the level of an epidemic.

Their worst-case scenario? More than 1.4 million cases by the end of January if governments stop what little they are doing now and if much, much more is not done to stop the spread. That’s assuming cases are being underreported by a factor of 2.5.

But those numbers are based on what was happening in August, Frieden said. "Events on the ground have changed quite a bit since," he told reporters. "We are seeing a rapid scale-up of the response."

The U.S. is sending in troops to help build treatment centers, staff to rapidly train local health workers and workers from elsewhere in Africa. The U.S. is also sending in tons of equipment and supplies and President Barack Obama appointed Maj. General Darryl Williams to coordinate efforts.

And if that response continues, the news looks encouraging. In Guinea, there are plenty of beds now and Guinea isn't included in CDC's scenarios. But in Liberia, there's still a critical need.

"What the modeling shows us is that even in dire scenarios, if we move fast enough, we can turn it around," Frieden said. "I am confident that the most dire scenarios will not come to pass."

"I am confident that the most dire scenarios will not come to pass."

“Once 70 percent of patients are effectively isolated, the outbreak decreases at a rate nearly equal to the initial rate of increase,” the CDC team wrote in their report.

"Even a week or two ago I would have expected things to look differently…than they actually look today," Frieden added.

The CDC scenarios differ from the WHO projections. WHO projects 20,000 cases in West Africa, but not until six weeks from now. It's hard to tell how many people really are being infected and what’s happening to them. Only a portion are even seeking help — many are hiding or quietly dying at home. And many more are being turned away from overwhelmed treatment centers.

Frieden pointed out that even basic care can save patients' lives. "The more that is known, the more patients will come in for care," he said.

"A surge now can break the back of the epidemic."

The hope is that the world, which has been very slow to help, is finally now beginning to act.

"Every hour counts. Every minutes counts. The data makes that clear," said Gayle Smith, senior director of the National Security Council.

“The U.S. government and international organizations recently announced commitments to support these measures. As these measures are rapidly implemented and sustained, the higher projections presented in this report become very unlikely,” the CDC team said.

The United States has announced a new push, response, sending troops and supplies and offering to help coordinate help. WHO and the UN are urging other countries to help. Germany, France, Britain, Cuba and China are among countries also sending teams and equipment.

"It's definitely still possible to reverse the epidemic and that’s why the initiative announced by the president last week and that we are seeing on the ground in Liberia this week are so important," Frieden said.