Could be demodex mange, fungal infection, allergies...the list goes on and on.
Get the vet to do a skin scraping...just a painless scraping to get some cells-like having a good back-scratch-and check for demodex and fungus first-before putting your dog through allergy tests.
Both demodex and fungal infections are transmissible through touch.
Demodex is especially contagious to animals in unsanitary conditions, and especially to pups from their moms, and dogs with thick fur or wrinkly skin, like Shar peis.
The treatment for demodex (a cigar-shaped skin parasite) is Goodwinal ointment-usually every four or six hours. The more you stick to the schedule, the better chance of getting rid of it.
Fungal infections smell yeasty and seems to come back almost immediately after a bath. Metronidozole (Flagyl) or other topical or oral antifungals are used.
Puppy mill dogs and shelter dogs can spread these ALL over the place.
Same thing with grooming shops. All you need is one dog with it, and it can spread to all the clients' dogs and yours, too.
Infection control is SOOOO IMPORTANT!!!
Love and Light-
"What is man without the beasts? If all the beasts were gone, man would die from a great loneliness of spirit, for whatever happens to the beasts soon happens to the man. All things are connected."-Chief Seattle