When we were kids we used to shriek "ew! boy/girl cooties!" and our parents would laugh. Eventually we grew up enough to realise cooties don't really exist, probably around the time we figured out all those other fictional creatures of our childhood are fake as well (except Santa - I still believe).
But today I was thinking... boys really do have cooties. And so do girls. Well, some of them do. Only now we call them other names, like "herpes."
Ah, the wisdom of children. We really should take them more seriously.