Aylward, speaking in a conference call with journalists, reiterated a comment made last week in Washington at a meeting on Ebola at the World Bank headquarters: This virus is killing 7 of every 10 people who become infected. That statement contradicted many earlier estimates that it was killing about half the victims.

The latest WHO numbers show 8,914 suspected or confirmed cases of Ebola and 4,447 deaths. That would seem to imply that half the people stricken with Ebola will survive the disease. But Aylward said that underestimates the true death rate.

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“We have had to carefully identify those individual patients for whom we could follow their entire course – when we do that carefully, we find that 70% are dying and that this number is pretty robust across the 3 worst-affected countries,” Aylward said in an e-mail to The Washington Post.

At current rates, there will likely be 5,000 to 10,000 new cases a week in Liberia, Sierra Leone and Guinea by Dec. 1, Aylward said in the conference call. To stop the outbreak, the WHO is pushing a plan to make sure that 70 percent of burials are safe and that 70 percent of sick patients are in treatment by then.

“That’s a real stretch target,” he said.

The trend in caseloads is “relatively flat,” with about 1,000 new cases a week for the past three or four weeks, Aylward said.

But the collection of data has been difficult in Liberia, Sierra Leone and Guinea, the three countries that have been overwhelmed by the outbreak, and Aylward cautioned against drawing any conclusion from the numbers.

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“Is the epidemic slowing down, or are we not seeing the exponential growth?” Aylward said. “Quite frankly, it’s too early to say.”

One of the epicenters of the outbreak, Lofa County in Liberia, as well as two districts in Sierra Leone, have seen a drop in new cases, he said. Officials believe this is the result of a “real change in behavior,” including changes in burial practices, Aylward said.

But the nature of Ebola requires that officials track down everyone who might potentially have been exposed. That’s the only way to keep it from flaring back up, which happened earlier this year after a quiet April and May.

It’s not good enough, he said, to have just a little bit of Ebola.

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“That’s like saying you’re only a little bit pregnant. This is Ebola. This is a horrible, unforgiving disease. You've got to get down to a level of zero,” said Aylward, who visited West Africa last week.

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He added, “Any sense that the great effort that’s been kicked off the last couple of months is already starting to see an impact, that would be really, really premature.”

Meanwhile, in the United States, public health officials have said they will change the safety protocols governing health-care workers after a nurse was infected while treating a Liberian man who had traveled to Texas.

"We have to rethink the way we address Ebola infection control, because even a single infection is unacceptable," Thomas Frieden, director of the Centers for Disease Control and Prevention, said during a news conference Monday.

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There have only been two Ebola diagnoses in the United States -- both occurring within the past two weeks -- but concerns have increased after Nina Pham, a nurse, was infected despite the protocols recommended by the CDC. In addition, the CDC does not know yet how many health-care workers may have been exposed to Ebola while Thomas Eric Duncan, the Liberian man who died of the virus last week, was being treated at Texas Health Presbyterian Hospital in Dallas.

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In a statement released Tuesday through Texas Health Presbyterian Hospital Dallas, where Pham works and is being treated, she said she is doing well.

"I’m doing well and want to thank everyone for their kind wishes and prayers," she said in the statement, which the hospital said it was releasing at her request. "I am blessed by the support of family and friends and am blessed to be cared for by the best team of doctors and nurses in the world here at Texas Health Presbyterian Hospital Dallas.”

Lena Sun contributed to this report.

[This post has been updated. Last update: 12:44 p.m.]