WHO's regional director for Africa, Mohamed Yakub Janabi, said at a press conference on Monday that sequencing showed transmission had been under way for several months when the outbreak was formally declared on May 15. Some early cases were incorrectly attributed to malaria and typhoid.
The outbreak is described as the fastest-growing Ebola outbreak ever recorded.
We are chasing the virus, the virus is ahead of us, said Janabi.
The number of confirmed cases in the outbreak has reached 4,200, including more than 1,900 deaths, according to recent figures.
