Fleas are the most common cause of skin disease in cats and dogs. Flea spit contains chemicals which stop blood clotting until the flea has finished feeding and these chemicals may cause an allergic reaction in your pet. Most animals are not affected by this allergy, but those which are suffer severe itching. Affected animals lick or rub themselves, wearing away their fur and making their skin red and sore. Sometimes a crusty rash will develop. Allergies appear most often in summer when the flea population is greatest. Skin problems may continue long after the flea which caused it has gone but they should eventually disappear if you treat your pet to remove fleas and continue treatment to stop the fleas returning. In the short term your vet may prescribe drugs to stop the itchiness.