My cats are all indoor and we have had fleas several times - they can come in on your shoes and clothing. Because we don't usually think to watch for fleas with kitties indoors - they can multiply pretty quickly before you notice.

I find that Advantage drops work like a charm. Usually one treatment is all it takes too. Fleas are mostly dead within a few hours. I order mine online at PetShed, but you can also get them at the vet office. That is what I would suggest