Skin Allergies and Itching in Dogs

The itching, the scratching and more often than not, there is a foul odor noted with your dog. They seem miserable and uncomfortable most of the time, often requiring multiple visits to your veterinarian.  You’ve checked them for fleas.  Skin infections have been addressed, but the problem still persists. What is the cause?  Why do they scratch and why do they always smell irregardless of multiple baths?