Primary problems include mites, such as scabies and demodex, flea allergy
dermatitis, food allergy
dermatitis, ringworm, atopy (environmental allergies), and others.
If your dog becomes itchy from one or more of the above and scratches and bites the areas, bacteria and moisture (from his mouth) are introduced into those areas. The bacteria causes a secondary pyoderma (bacterial skin infection), and the moisture sets up the right environment for a yeast pyoderma (yeast skin infection).