Unfortunately in order to eradicatate fleas one must become familiar with their life cycle:
http://vetmedicine.about.com/od/para..._fleacycle.htm
Paraphrased in a nutshell:
The adult flea uses the blood of the cat or dog to reproduce, they will bite humans but they cannot lay eggs from human blood. The adult flea lives on the animal and lays it's eggs there. The eggs however fall off in the environment (house). The female flea can lay up to 20,000 eggs in it's lifetime.
The eggs then hatch into the larva which lives in the environment living off of organic debris and flea feces (digested blood - the little black flecks you see in the pets fur), they then cocoon into the pupa stage where they can remain in this stage from a few days up to a year or more as they require the right conditions in order to hatch (humidity, temperature, carbon dioxide levels, motion...).
Flea larva do not like direct sunlight therefore they will not be concentrated in the center of your carpet in a sunny room, they will migrate to cooler dark shady areas such as around baseboards, under/in furniture and other cracks and crevices.
Most household fumigation products only have a "kill action" of two weeks therefore it is important to spray every two weeks until the problem is under control. You need only spray the areas mentioned above. Do not spray and then go on vacation for two weeks because this will change the living conditions and the pupa will not hatch until you come home, then.....whamo you have a big problem on your hands again!!!
Some products have the 2 week kill and then also have another ingredient a hormone called precur or methoprene which acts as "birth control" for fleas rendering any eggs that are laid as not viable therefore cutting off the cycle at the egg stage. These are usually household sprays, animal sprays, mousse etc.
Some products only act as birth control. The pet products Program and Intercepter act this way therefore they will not kill the adult fleas, but will cut the flea lifecycle at the egg stage. These products are best used BEFORE you get a problem as they will prevent the problem from getting out of hand. These products are edible products given internally to the pet.
Products such as Advantage, Advantage multi, Revolution, Advantix etc. are TOPICAL products put onto the skin of the pet once a month and are used to kill the adult fleas.
Animals that do have fleas it is also recommended to treat the animal for tapeworms as there is one species of tapeworm carried by the flea and your pet will ingest the flea through normal grooming and therefore develop a tapeworm.
Because the flea pupa can remain dormant for a year or more, it is recommended to keep treating for at least 12 months if you have a serious problem. If you stop at 2,3,4 months etc. You might be surprised later when you are having to repeat the whole process all over again.
If you don't have a flea problem then if you get whatever product you choose to use (they usually come in pkgs of 6) and begin treatment the first of May with the last taken the first of October. Your pet will be treated until the end of October (when the frost comes) and fleas should no longer be a problem.
Always use caution is choosing the right product!! I cannot stress this point enough!!!! Some products are EXTREMELY toxic to cats. Most products contain Permethrin and or Pyrethrin which comes from the Chrysanthemum plant, these ingredients are deadly to cats.