Hi Connie.
cúagusgiorraà is correct, it is inherited, in fact every dog born had the demodex mite. However it is more apparent in pups, and will often flare up in times of illness and stress in the dogs life.
If you have only recently had the dog, this could be the reason, also she is of the right age. Yes it does take a long time, and it may come back at some point, but certainly not always and most dogs are 100% agterwards. Dogs that have shown evident Demodex mange should not be bred from either.
Dawn.