Maybe instead of swine flu, there's been a global outbreak of stupid.
I mean, when the slayers realize that they're being tracked by their magic, they get rid of it with no easy way to reclaim it (instead of say, luring their trackers into an ambush, or finding a way to sent false positives or, ya know, block it. What is Willow good for, exactly?), then stay in one large, powerless group and to top off the stupid, shack up with a known associate!