The World Health Organization is pleading for a global investment of nearly $1 billion to fight the spread of Ebola, warning that the amount is merely the minimum needed to keep infections of the deadly virus "within the tens of thousands."

The organization has long warned that Ebola's toll — which stands at at least 2,461 dead and 4,985 infected as of Sept. 13, according to the WHO — vastly underestimates the crisis. But now it appears that the epidemic is spreading at such a rapid pace that tens of thousands more infections and thousands more deaths are expected, even with a dramatically scaled-up response.

The toll has doubled in the past month, the WHO's Assistant Director General Bruce Aylward said Tuesday at a news conference in Geneva, according to Reuters.

The WHO warned that the number of cases could start doubling every three weeks, the Associated Press reported.

"Quite frankly, ladies and gentlemen, this health crisis we're facing is unparalleled in modern times," Aylward said. "We don't know where the numbers are going on this."

The epidemic sweeping across five West African countries — and most severe in Sierra Leone, Guinea and, especially, Liberia — has likely claimed many more lives than the official numbers confirmed by the WHO.

That's due in part to the limited resources in place to fight and document outbreaks of the disease, which has devastated the infrastructure of already strained health-care facilities in the affected countries. In Liberia, transmission rates are increasing exponentially, and health-care facilities are filled to capacity and are turning away many new patients.

The global response to help those countries fight the outbreak has been chaotic, as The Post recently reported. Many global health groups capable of providing assistance to the region underestimated how severe it would be in the outbreak's early days, and have scrambled to catch up to the needs of those working to fight it. And with the outbreak becoming a full-fledged epidemic, Ebola aid workers are now overwhelmed and exhausted.

On Thursday, the U.N. will announce a global response coalition against Ebola. Valerie Amos, under secretary general for Humanitarian Affairs and Emergency, said that international groups will need to "prevent the collapse of the health systems in the affected countries" and contend with a shaky "food security situation" in many regions affected by the epidemic.

On Tuesday, the United States will announce its own plan to aid in the fight, led by the U.S. military. The operation, which President Obama will announce at the Centers for Disease Control and Protection in Atlanta, will involve 3,000 troops and could cost as much as $750 million over a six-month period. Operation United Assistance, as it's called, will be headquartered in the Liberian city of Monrovia.

In the rapidly escalating war against what Liberia's finance minister recently described as "an enemy we don't see," the U.S. military will set up 17 treatment centers in Liberia. The United States is also planning on training hundreds of health-care workers to help in the fight against Ebola.

"Our collective response to date has not been sufficient," Samantha Power, U.S. ambassador to the United Nations, said Monday of the global response. "The situation on the ground is dire and is growing worse by the day."

RELATED:
U.S. military will lead $750 million fight against Ebola in West Africa

A virus hunter faces the big one: Ebola

Will the Ebola virus go airborne? (And is that even the right question?)

'It’s despair on all fronts,' says Ebola aid worker

As Ebola cases accelerate, Liberia’s sick must fend for themselves

Ebola's deadly toll: Portraits of grief

This is how people are being warned about the Ebola epidemic in West Africa

Suiting up for Ebola