We've had only one problem with fleas and luckily that was a few years ago now. It was extra annoying in our case because the cat never *ever* goes outside and is never around other animals that may have fleas to transfer... so it was quite a mystery until we talked to her vet.
The vet told us that people don't often realize that fleas can be brought in on human shoes, pant legs, socks, etc. As soon as I heard that, I understood... there are rabbits and squirrels in our yard constantly and I often see them scratching.
Until this happened, I didn't think we had the need for flea medicine, but since those critters are out there and we walk around there, we've had to use the flea drops during the warm months.