There is about 1% chance of having a cheater in your game, like most people mentioned they are there but it's still rare that you find them and those who get banned also get a hardware ban, and although there are workarounds to it, not all cheaters will know how to use it.
Cheaters are more prevalent than they've ever been.
It's also easier to cheat than it's ever been.
Lots of the Chinese/Asian cheaters cheat in Gaming/Internet cafe's. (Literally thousands of these around)
If they end up with a hardware ban, they just move over to a different machine/shop.
There was a detailed post on steam a while ago where a guy had pics and detailed exactly how it was done.
It's pretty much an all access pass to cheating. Nothing can really be done about it. That's why they've banned over 500 000 players and they're still banning thousands daily.