The Vet said it most likely came from the breeder.
Well, not exactly from the breeder

From the pup's mother, most likely, or possibly any other dog your pup has come into contact with. Demodex mites live in the hair follicules of every dog on the planet, and populate the coats of newborns within a day or two of being born. If that is not clear, it means that every single dog past his first couple of days of life has demodectic mites ALL of the time. They are a natural flora of the dog's coat.
They are NOT transferable to humans. You have your own variety inhabiting your hair follicules (which don't choose to live on your dog). If you've read somewhere that they may be transferrable, I'd suggest that either you or some writer has confused demodectic mites with sarcoptic mites - which cause a different sort of mange (sarcoptic mange). Sarcoptic mites are scabies - and they ARE transferrable. Doesn't matter to you though, you're talking about demodectic mites, they're normal on your dog, and they are not transferable to you.
With that out of the way, dogs - especially puppies - do get demodectic mange. This happens when the dog's immune system is not functioning properly, and is unable to keep a balance with the mites. Too many mites = damage to the hair follicules, and then you get hair loss. In the case of most puppies, what this actually means is that the still-developing immune system just isn't yet mature enough to cope with the massive stresses that get placed upon it. Which really isn't that surprising - we bombard young pups with everything from the stress of rehoming, multiple deworming treatments (pesticides), tick and flea treatments (insecticides), and a plethora of vaccinations. All of these are a massive hit to the immune system, and the poor pup is only 2-3 months old yet. An adult dog with a fully developed immune system would have trouble coping with that, and a baby with his still-developing immune system easily gets overloaded. And in that situation, mites are just one thing too many for his immune system to cope with. Result can be mange.
The GOOD NEWS is that 90% of puppies will recover perfectly well from mange without any treatment whatsoever - all they need is a bit of time and a bit of peace for their immune systems to develop a little bit further. So if your puppy is showing any signs of hairloss, all you have to do is STOP the assaults to his immune system until it grows a bit stronger. That is, do NOT give him any more vaccinations, no more tick/flea treatments, certainly no elective surgeries until all signs of mange are gone. Feed him the best quality diet you possibly can - nutrition is the underpinner of good health and immune system function after all - and generally just try to limit the stresses he has to cope with until he has recovered.
What your vet may be talking about, incidentally, with the hereditary factor is not the mites themselves. As mentioned, every dog has them, that's just simple fact. What every dog does
not have though, is a weak immune system that has trouble coping with the natural presence of demodectic mites. And its that weak immune system that is hereditary (and excludes animals from breeding). Now your puppy
might not have a weak immune system either. He may very well - as is the case for
the majority of puppies with demodectic mange - just have an as-yet undeveloped immune system. If that is the case, he will not develop more than a few small spots of hair loss and he will recover from it all on his own (as above, with stresses to his immune system put on hold for a bit). It is really only 10% of mange sufferers who go on to develop generalised mange (more than just a few spots) and who require treatment to help them recover. THOSE few individuals can truely be considered to have dodgy immune systems and need to be excluded from any breeding programs, as weak immunity is frequently hereditary.