The little critters were an accident, to be fair. They were supposed to be a new weapon against disease. One sting, and their venom could cure practically any disease or ailment. While it did that, no one could have predicted the side effects. Of course the government had to shut it down. The consequences could be catastrophic if these bugs got out.
If only they hadn't used bugs, maybe their containment might have worked. But the small creatures could very easily slip through cracks or, in our case, an unseen vent that led directly to the outside world. A world that would never be the same.
The bugs swarmed through their newfound scenery for a while before splitting up. Some found local flora and began pollinating plants while injecting them with their venom, changing the plants' primary goal.
Others found other bugs they could reproduce with to make more variants of the new species. Mutated forms of previous bugs caused different changes in the venom, which would cause different changes in people when they were exposed.
Some bugs weren't as fortunate, learning the harsh reality of the wild. Deserts cause thrist, water drowns, snow freezes all over and fire consumes indiscriminately. Those that survived those trials were left stronger than brethren, and their venom showed it.
There were a few, however, that got lucky. They managed to find the species that they were born to change. They managed to find civilizations that would never be the same, for better or worse, after the bugs appeared. There were more than a few places they could go -- colleges, companies, airports and parks -- the possibilities were endless.