Honestly? This is Texas, and I've been opening the windows a lot when I get home at night while the weather is nice. All Foot really has to do is sit next to the screen: no doubt the fleas got in through the screen mesh. And yes, we have other cats outside...and squirrels.

Foot was clean when she went to the vet--I had her checked for everything--and it was too cold yet for fleas.

Which is one reason why I hate Texas. It is, seriously, Annoying Bug Pest Capital of the World.