Video: What Pet Owners Need to Know about Dog Flu
The first answer is pretty thorough but I would like to add a few points.
First.. the type of food you are feeding your dog may be the key. If you are using a food that is grain, rice or corn based in the first three ingredients, please change to a food that is meat or chicken based. Many dogs are allergic to grain, rice, wheat and corn and these things are used as primary ingredients in many well known, name brand dog foods. Secondly, all treats that have these ingredients should be cut out. It takes weeks to months before a food change can show results so you must be patient. Jumping from one food to another will be counter-productive.
Some dogs are so allergic to flea or other insect bites that one or two bites can cause a severe reaction. Once the itching/scratching cycle begins, it is hard to stop it. Benadryl is good but I suggest liquid form. You can get it inexpensively at one of the dollar stores. The pills take time to break down in the digestive system so you get better and faster results with the liquid.One tsp= 5cc and most dogs of average size can tolerate that much and more if the dog is a larger breed.
Allergy testing is very expensive and most dogs will test positive to some things. That does not mean that the itching is coming from those things as many things in the environment can trigger positive test results. Breaking the cycle of itching is primary and that can be done with prednisone ( short term and last resort after benadryl and bathing), allergy shots( same deal, last resort after easier methods). This spring and summer have been particularly hard, weather-wise.