Fleas are hardy and determined. Additionally, the fact that fleas don't spend most of their time on the pet makes the problem of locating them and wiping them out harder still. Their most resilient stage, the tiny hibernating cocoon, can live in your environment over a year without feeding and is protected by an impenetrable shell. It is this stage that survives most treatments and returns to breed and re-populate year after year like a cruel sequel to a horror movie. It's something no one wants to know.
Pets act like small mine- sweepers, collecting hatched and hungry fleas from everywhere.
Treating the pets is essential for their own comfort as well as to kill any errant fleas entering the area from afar. White clothing, socks, and sneakers excite and invite them right into the livingroom where they remain unseen and happily reproducing without your ever knowing until the problem is advanced. Treat your entire square footage indoors including garages, outbuildings, and dog houses or kennels. You simply cannot allow a single flea to survive in your or your pet's, environment if you want to live a life free of allergies, disease, and irritation. The yard is the most arduous task, but you MUST treat the area outdoors with just as much determination as you treat the pets and house.