I Hate Fleas!

Does anyone know of a good reason for fleas to exist? We have been battling them at our house all summer. I blame the cats since they are the ones who spend most of their time outside. We got a bit behind (read: lazy) about giving the pets their spot flea medicine, and before we realized what was happening they were everywhere. It is similar to dealing with head lice, for those of you who've had the pleasure. Treat the animals, vacuum and wash sheets daily, and brush them to get any stray fleas out. I hate it! Fortunately it seems to be getting under control, but it is a huge robber of time and energy. Oh, well. At least I've got more time to deal with it while we are off for the summer.