From what I understand, this is not rare. I assume you don't live in a condo. People can bring in fleas home on their shoes or clothes. Fleas can jump through window screens, if your cat is sitting by an open window. Your cat can pick up fleas at the vet, if they caught a mouse (a whole other problem!), if someone brings a dog to visit, etc. Hopefully Frontline should prevent this from being a problem?