Here is a flea emerging from its cocoon. I'd go through the whole flea life cycle, but these folks have done a lovely job for you. The point is that most of the flea's life cycle takes place OFF the animal in the environment. Each flea lays hundreds of eggs each weak, and they fall off into the environment. Once they develop into a new flea, they can remain dormant for months in your yard, carpet, upholstery, etcetera. This is why having a flea-control product that stays on the pet for one month is so great. When they return to the flea-cocoon-infested area, and pick up new fleas (within seconds), the new fleas die before they start the cycle over again.
Unfortunately, this only works if the product really stays on the pet for the whole month, if that's what you're expecting it to do. Today we had a lady who felt that her dog must have multiple allergy problems requiring lots of cortisone, because she had the fleas under control -- she was using Advantage every month. Examination revealed that the dog had many fleas present. "How often do you bathe him? Bathing removes the Advantage pretty quickly you know." Turns out, this dog likes to roll in nasty things (imagine that!) and sometimes is bathed more than once in a single day. He certainly gets several baths per month. This is one of those times that points up why it is so important to take a complete history when looking at the dog's problem. You need to examine that patient's entire lifestyle.
Now Adantage (Bayer's imidocloprid) is a good product. It spreads over the entire body when you squirt the little tube on the back. The chemical forms tiny crystals that stick to the hair and skin and kill fleas (and their eggs) on contact. A simple wetting, like getting caught in the rain, does not remove them. It does last for a month, unless... Scrubbing with soap and water takes it right off. The first bath will take half of the product and the next bath takes the rest. The dog is now "Advantage-free" and unprotected as far as fleas go. If you're bathing three times per month (much less three times per day), this is not a good product for you to use once per month.
The over-the-counter products you can buy at Wal-Mart are either pyrethrin or permethrin. They are also water-soluble, so a bath eliminates the product. Worse, though they say you can use them once per month, they don't mention that these particular chemicals degrade rapidly in the open air and sunlight. If you use these products, you probably need to use them once per week, not once per month. Now they don't seem like quite so much of a bargain. [Note: the permethrin-containing products say "do not use on cats"; this is no joke -- it will kill them.]
At KVC, we've had better luck with Frontline Plus (Merial's fipronil). Like the aforementioned products, you apply the contents of one pre-measured tube to the pet's skin, putting it under the hair. Unlike the aforementioned products, it really lasts a month for most pets, and it survives bathing. Fipronil is oil-soluble, not water-soluble. When you apply it, it distributes through the skin oil, spreading over the entire body. The excess wicks down into the oil glands in the hair follicles. Certainly you can scrub it off with soap and water. You know how dry your own skin feels when you get out of the shower in the wintertime -- you've stripped off the oil layer and there's nothing to hold the moisture in your skin. Even without lotion, the skin will put out another layer of oil from the little glands in the hair follicles. If the pet has had Frontline applied, that skin oil will contain the flea-killing chemicals. Even bathing once weekly, you should still have pretty good flea-control for a month.