If your critters go outside at all - even if just to take care of business and right back it - they will pick up fleas from the yard if you don't treat your yard.... And if you have 'visiting critters' you will have fleas and lice and who knows what all... the vet told us that if you have squirrels, etc you WILL have fleas.....
Being as how we live in the land of a gazillion bugs my DH sprays around the perimeter of the house every month with malathion that he mixes in a spray thingy = can't think of the word right now..geez We also use stuff for fire ants and that helps.
We also use Frontline Plus (the Comfortis pills or whatever they are allowed our dog to get a TICK! which made her sick until we finally found the bugger) and the Frontline kills the eggs, mosquitoes, etc etc..