Fleas are not just annoying, but potentially deadly to both pets and humans. In addition to leaving painful and itchy bites, they can transmit a variety of diseases, including cat-scratch fever, even typhus to humans. Animals, on the other hand, can get tapeworm and other more harmful illnesses.
And if you have one flea, you probably have, well, lots of them -- for every one flea you see, there can be a few hundred eggs and larvae waiting their turn. Indeed, flea infestations can very quickly go from bad to worse.