New projections from the World Health Organization paint a worsening future for the months ahead if Ebola is not properly contained, suggesting that the caseload will total more than 20,000 by November. The new data also unseats the common perception that Ebola has a disproportionate impact on women, instead finding that the epidemic has affected men and women in roughly equal numbers.

A separate analysis, from the U.S. Centers for Disease Control and Prevention Tuesday, projects the caseload could be even higher—between 8,000 and 21,000 by the end of September in Sierra Leone and Liberia alone. The CDC calculations assume a vast number of cases have been underreported. A scaled up Ebola response, rolled out by the White House last week alongside other international aid, may render the devastating projections from both groups less likely. The new figures, however, will help public health experts better understand the spread of the disease, who is getting sick and what it will take to reverse the trend of infections. “It’s definitely still possible to reverse the epidemic,” says Tom Frieden, the director of CDC. The CDC model comes from a new online tool that responders in west Africa could use to model what might happen in various Ebola scenarios.

“Today the modeling coming out from WHO and CDC certainly show what can happen if we don’t inject a large bolus of international assistance,” Beth Cameron, a U.S. National Security Council staff member, said at an Ebola conference held at Georgetown University today. The U.S. response is “going to make a big difference,” Ellen Johnson Sirleaf, president of Liberia, said in video remarks at the Georgetown conference. 

The new numbers come six months after the WHO first published formal notification of the Ebola outbreak in Guinea on its website. The disease is spread by personal contact with the bodily fluids of patients, often in a caregiving setting or during traditional funeral rites. So far there have been 2,803 deaths in this outbreak, with 5,843 cases. Among the patients were 337 health care workers. More than 181 health care professionals have died.

Reported Ebola cases in Liberia have been doubling every two to three weeks and those in Sierra Leone double every 30 to 40 days, CDC reports in its Morbidity and Mortality Weekly Report. WHO’s analysis, in the New England Journal of Medicine, also finds that to date, Guinea, Liberia and Sierra Leone have an Ebola death rate of about 71 percent—much higher than earlier projections that totaled around 50 percent. In hospital care settings where patients receive electrolyte replacement and rehydration, however, survival rates are higher.

The epidemic, which now has a firm hold on cities in Liberia and Sierra Leone, is more challenging to stamp out than prior outbreaks that raged in isolated areas and burned themselves out, says Daniel Lucey, a professor of microbiology and immunology at Georgetown University Medical Center, who recently returned from caring for Ebola patients in Sierra Leone. “This time the control methods are not effective, and in my own opinion they’re not going to be effective in Monrovia [Liberia] or Freetown [Sierra Leone],” he said at the Georgetown Ebola conference. “It’s something different. It’s urban Ebola.”

WHO Tuesday also put out country-by-country reports on Ebola. Liberia, where the cases are continuing to increase exponentially, has the most formidable Ebola obstacles and highest death toll. During one 24-hour period last week 113 new cases were reported, and 14 of the country’s 15 counties now report cases. WHO projections suggest that Liberia could have 9,890 cases by November 2—dwarfing the projected caseloads in Sierra Leone or Guinea, which together would total about 11,000. “Without drastic improvements in control measures,” the report concluded, “the number of cases of and deaths from [Ebola] are expected to continue increasing from hundreds to thousands per week in the coming months.” Moreover, the Ebola epidemic has severely hampered daily life and economic activities in Liberia. “We expect the economic impact to be severe,” Sirleaf said.

Yet there were some positive notes in the international assessments. Nigeria and Senegal’s caseloads, for example, are both stable. Senegal has had no new cases during the past three weeks and if it remains Ebola-free for the next 21 days WHO can declare its outbreak over. Nigeria, meanwhile, has limited its outbreak to 15 confirmed cases in Lagos and 4 in Port Harcourt, without any further spread. “If Nigeria can control an outbreak caused by such a deadly and highly contagious virus from the start, any country in the world can do the same,” WHO wrote.

Still, the epidemic, which has rapidly outpaced available health care resources, is also hard to track accurately and no one can predict with certainty how the outbreaks will evolve. Even knowing the true caseload and finding all patients remains challenging. In Monrovia, Liberia’s capital, “the true number of deaths will likely never be known,” WHO wrote—some of the bodies have simply been thrown into the two nearby rivers.