I have always been wondering where do special infected come from. I mean, ok, zombies are infected humans, but how were the special created? I seriously don't now if there is an official article by Valve, explaining this, but if not, I want to imagine a story :D All I can say is that they were exposed to radiation or were mutants before the infection and this caused this kind of mutation after the infection. But still, this theory can somehow explain the Hunter, Smoker or even the Witch, but it can't stand for the Tank, the Charger, the Boomer or the Spitter. That's just my opinion so leave a comment below with yours or just corect me if there's an actual official explanation about them by Valve.