There are fears the deadly Ebola virus could mutate if it is not brought under control soon.
Dr Thomas Kenyon, director of the US Centers for Disease Control and Prevention (CDC) said the epidemic is now "spiralling out of control" and the window of opportunity for stopping it is closing.
He said: "Guinea did show that with action, they brought it partially under control but unfortunately it is back on the increase now. It's not under control anywhere."
"The longer the outbreak goes uncontained, the greater the possibility the virus could mutate."
While Ebola is transmitted in humans by contact with bodily fluids of the sick, suspected cases of airborne infection have been reported in monkeys in laboratories.
UK trials of a vaccine for the Ebola virus began today - but some experts complain that the response has been a long time coming.
A Swansea doctor who took leave to treat Ebola patients in Africa, is back on the wards after three weeks in isolation.
The first British case of Ebola in the outbreak in West Africa has been confirmed. So how would repatriation work are what are the risks?