Even if you do not have pets, it is more than likely that a rat, a ferret or some kind of mammal host had also resided in your home at some point in time. Wild ferrets or rodents are known to settle in attics or ceiling areas and the fleas on them could fall or crawl in through the tiniest gaps in your ceilings , walls or even light fixtures, thus infesting your house.
First of all, you have to understand that most flea problems aren’t usually concentrated at just one particular location of your house. If you have pets at home, it is more than likely that they’ve “spread” these fleas everywhere they’ve gone to. A dog or a cat is basically a 24/7 operating shuttle for these fleas to hop on and off wherever they want to. Wherever your dog or cat has been to, it is more than likely you have fleas there as well.