From what you say in your question, I would surmise your one or more of your cats has or have a bad case or bad cases of tapeworms. They get these from eating fleas that are carrying the parasite (tapeworms). You need to get rid of the fleas as well as the tapeworms. If you have other pets (mammals), you need to eradicate fleas on all of them. Just get rid of the worms and the cats will just get them again next flea they eat. The cat doesn't go to eat the fleas, this happens because cats sometimes scratch flea itches with their teeth and they clean themselves with their tongues. Take your cat to the vet to be treated for fleas and tapeworms. Take a stool sample to be sure it is indeed tapeworms. You only need a piece the size diameter of a nickel, but make sure it is fresh (within the last 8 hours, if possible). Until the fleas are completely eradicated from cats and house, keep checking periodically for recurrences of tapeworms. If they were my cats, I would give each a Capstar tablet to kill the fleas that are on them right now. One Capstar should do it within the first hour or two of taking it. Also I would treat them with Frontline. You can do both at the same time. Frontline given once a month will keep the fleas from coming back to the cat. Make sure the doses of all medications match the weight and type of pet you are treating or you may do more harm than good. If the fleas in the house get feisty and start attacking you (humans) I would have the house treated. If not, I wouldn't worry about it. If your pets are all properly treated for fleas (especially by Frontline, Revolution or some other veterinarian recommended treatment) and the fleas aren't bothering you, they will eventually die and never return. I would stay away from grocery or drug store topspot treatments as they have been known to have very nasty adverse affects with pets. I've even heard of pets dying from these affects. They have a bad reputation for bad affects on pets and, I'm afraid, they aren't effective enough to outweigh the odds of your pet being at risk ...or the money you may be trying to save.
Note: If you have any Collies or related breeds of dogs, ask the vet about which flea treatment they should get. Collies often have bad reactions to topspot type treatments.