GENEVA (AP) — The Ebola outbreak in West Africa eventually could exceed 20,000 cases, more than six times as many as are now known, the World Health Organization said Thursday.A new plan released by the U.N. health agency to stop Ebola also assumes the actual number of cases in many hard-hit areas may be two to four times higher than currently reported. If that’s accurate, it suggests there could be up to 12,000 cases already.Currently, about half of the people infected with Ebola have died, so in the worst-case scenario outlined by WHO, the death toll could reach 10,000.“This far outstrips any historic Ebola outbreak in numbers. The largest outbreak in the past was about 400 cases,” Dr. Bruce Aylward, WHO’s assistant director-general for emergency operations, told reporters.He said the agency does not necessarily expect 20,000 cases, but a system must be put in place to handle a massive increase in the numbers.Separately Thursday, the U.S. National Institutes of Health announced it will start testing an experimental Ebola vaccine in humans next week.