4 years ago I had an indoor only cat who got fleas. The vet said that we must have carried them it. They jump onto anything coming into the home. We stopped walking across the grass to go to the mailbox just in case they jumped back on us.
Each time you vaccumn make sure that you empty it. Take the vaccumn and empty it far away from the house. Also, make sure you give them a good flea bath.
Good luck.