I got the worst flee infestation in my house about 3 years ago, the funny thing was i had no animals at the time, i sprayed my house and still it was getting worse. It was that bad that when i put a cup down on my carpet the rim of the cup would be covered in flees. I could not understand why.
I went for a coffee at my neighbours house, and she also had no animals and had the same problem, it turned out that a mutual friend of ours who had loads of cats ( that clearly were never flee treated) :wacko:had been bringing her flees in on her clothes. Her house was actually not as infested as ours, as apparently the flees will jump on the cat and then get dropped off outside, where in our non animal houses they just kept breeding with nowhere to go.
I ended up gutting my whole house it got that bad which did solve the problem, and my friend and i had to ban our mutual friend from our house as did her own mother. I would be asking yourself what people come to your house, as it could be them bringing them in. Now i treat my house regularly for flees as once you get that many its hard to get rid of them