The worst Ebola outbreak in history is only getting worse.

The World Health Organization said on Thursday that the outbreak that has ravaged West Africa could infect 20,000 people before it is over. With 1,552 deaths recorded so far, the virus is certain to claim more lives than all of the previous Ebola outbreaks combined.

At least 3,069 people are believed to have been infected in the current outbreak, but WHO officials believe that the real tally is far higher.

“[I]n many areas of intense transmission the actual number of cases may be 2-4 fold higher than that currently reported,” the organization said. It added that “the aggregate case load of [Ebola] could exceed 20,000 over the course of this emergency.”

The organization released a “roadmap” for stemming the rising tide of infections within six to nine months.

The new plan of action comes as the outbreak appears to be getting worse. Forty percent of infections included in the current tally occurred within the last three weeks, WHO said Thursday. And the virus has mutated during the outbreak, which could hinder diagnosis and treatment of the disease, according to scientists who genetically sequenced the virus in scores of victims.

U.S. government researchers, in collaboration with British drugmaker GlaxoSmithKline, announced Thursday that they will begin human trials next week at the National Institutes of Health in Bethesda, Md., for an experimental Ebola vaccine. Health officials said they want to rush the drug as quickly as possible to health workers and others at risk in Ebola-ravaged West Africa.

In Liberia, where international health organizations have said the outbreak could be spiraling out of control, Centers for Disease Control and Prevention Director Tom Frieden said the situation is far worse than he anticipated.

“It’s even worse than I’d feared,” Frieden told CNN. “Every day this outbreak goes on, it increases the risk for another export to another country.”

The country has experienced the brunt of the outbreak, and already weak health systems are buckling under the burden of over 1,300 cases. Violent clashes with residents and police broke out in Liberia’s West Point slum, fueled by fear of infection and rage that the entire community had been forcibly quarantined in an effort to prevent the spread of the virus.

So far, cases related to the Zaire strain of Ebola have affected only Guinea, Liberia, Sierra Leone and, to a lesser extent, Nigeria.

On Wednesday, Nigerian officials announced the first Ebola death outside of the populous city Lagos, were several patients who came into contact with a Liberian man, had become affected.

The new case involves a doctor in Port Harcourt. His wife has been quarantined, and 70 other people they may have come into contact with are also under observation according to the BBC.

A separate Ebola outbreak — unrelated to the outbreak in West Africa — was recently reported in Congo.

Making the outbreak more acute in West Africa is the continued infection of health-care workers. According to WHO, 130 health workers have died from Ebola out of 225 who have been infected.

Two American aid workers, including one doctor, were flown to the United States for treatment after being infected in Liberia; both survived and were released from an Atlanta hospital last week. A British health worker who tested positive for the virus was flown to London on Sunday for treatment.

On Wednesday, the CDC said that a researcher was being flown back to the United States after coming into “low-risk contact” with someone sick with the disease. That person is not infected and is experiencing no symptoms.

Though the West African outbreak is far from over, already there are concerns that a food and sanitation crisis is on the horizon. The WHO noted that once the rate of infections has been brought under control, an additional plan will be needed to deal with those issues, as well as an increasing number of children orphaned by the virus.

Brady Dennis contributed to this report.

[This post has been updated.]