Or possibly Wesker's not in on this little outbreak, given that he isn't supposed to have a real Master Plagas sample and Ada's other employers do..and that other employer may be Tricel. But if he isn't behind the outbreak it is somewhat possible that he tried to gain control of the town and they turned against him, and so he manipulates Chris and the BSAA to go in and cause a big enough distraction that he can get in and grab whatever it is he needs with none the wiser he was there.
It's the sort of thing he tends to do. I always wondered who tipped off Leon about Ashley's location...if it was mentioned in RE4 I forget.

