External parasites range in importance from irrelevant to irritating to life-threatening. Our cat and dog's most common external parasite, the flea, is happy to have meals off us as well as our pets, although it really prefers feline or canine blood. We don't like fleas biting us. Moreover, the ubiquitous little flea is without doubt the greatest single cause of medical skin conditions in cats and dogs. And skin problems account for as many visits to the London Veterinary Clinic as any other single condition. Fortunately, most external parasite problems are relatively easy to diagnose and treat.