I realize that my answer isn't very environmentally friendly, but call an exterminator. We had a bad infestation recently, and had no clue where they came from. Our 1 cat has never been outside! We bombed EVERY weekend, and still couldn't kill them damn things! One visit from the exterminator, and they were gone. They came back in 2 weeks for a follow up, and will be back out in 2 more weeks to start our bi-monthly visits. I tried EVERYTHING! I washed my cat in Dawn, then washed him in vinegar to try and change the taste of the flesh. Didn't work. I bought Advantage, didn't work. We have all hard wood floors, with only area rugs, and they were everywhere! Our vet said that because of the poor winter, that we can expect a bad bug season,and people who normally don't have a problem are having them now. I even tried vacuuming every day, then spraying the floors down with lavender water, as they supposedly don't like the taste of lavender. With a baby on the floor, crawling around and getting bitten by the fleas, they had to go, and an exterminator was the only way that was going to happen. Good luck to you.