Actually that was apparently a Team Rocket thing. I guess they were trying to like...give themselves powers or something? That's about the only thing that makes sense to me in retrospect. They captured Suicune at one point, which pissed off Raikou and Entei and the weather pretty much flipped its shit for a few days because of that. Then like a month later people wound up mutating into like...mini Johto beasts. And had powers to go along with it.

[Let him tell you about being a level five, John. Let him tell you how much it sucked.]

And there were some Rocket attacks alongside the outbreak too, on some of the gyms and the centers and stuff. I think basically their plan backfired. I don't think they meant for it to infect the general population.