We got my mother in laws cat last April, and she brought in fleas with her. We treated the five cats we had with Advantage, and it worked great. We stopped in January or February because there was no signs of fleas. When May came, we noticed fleas on the cats again, and started the Advantage again, this time on the three cats we had left. It did absolutely no good, I was using a flea comb and getting upwards of 50 fleas daily. Now we are down to 2 cats, and still no luck. I called Bayer, they said to use flea meds more often, every three weeks. I applied some 9 days ago, and yesterday used a flea comb and got 80 off of only two cats. Is it possible the cats have built up an immunity to the Advantage and it will no longer work? Is there a certain med that has great results? I hate chemicals and really hate to flea bomb my house if there's another way to eliminate them.