The WHO warned on Thursday that the number of Ebola cases could rise to 20-thousand, as the infection is spreading at an uncontrollable speed.
The virus has already killed more than 1,500 people, and there are at least 3-thousand reported cases as of now.
But the health agency believes the actual number could be up to four times higher than that.
A high-level official at the WHO says the agency has been caught off guard by the scale of this outbreak.
"that is not saying we expect 20,000 cases, that is not saying we would accept, more importantly, 20,000 cases, I think that is completely unacceptable."
The WHO is calling for a full-scale humanitarian response to the crisis.
And that's going to require a lot of money.
"Going forward this is going to cost, we estimate, 489 million dollars over the next six months. It is a big operation, we are talking, well over 12,000 people operating over multiple geographies in very difficult and high risk circumstances, it is expensive."
There's some hope on the horizon though.
An experimental Ebola vaccine developed by U.S. and British-based health institutes will soon be tested on humans.
Tests have already proven successful on monkeys, and the initial human trial will be conducted on around 20 healthy adults at a National Institute of Health clinic in Maryland next week.
If everything goes well, the second phase is expected to take place in Africa, after strict scientific and ethical standards are met.
Kwon Soa, Arirang News.