There's the ones they know about, and surely a hell of
a lot more they DON'T know about. There's not enough
organization in Congo to keep track of everything. Once
a certain critical mass of infected exist in the city the
virus will blow up into a pandemic.
I know they have a vaccine ... but nobody seems sure
it'll work with every Ebola strain. There's also likely
not nearly enough of it for outbreaks in larger cities.