As far as I understand (trust me, I am by no means an expert here), but all dogs have the demodectic mites on them and Mom passes those few mites on to pups somehow (don't ask me how) and most pups fight them off and win, but there are a select few whose immune system can't fight them off and then the dog is prone to it for its whole life. I need to go and do some more research on it, but that's what I know.