How can fleas live in a vacant house for 4 months? They are infested.
If you read our Flea Control article, you’ll learn that flea pupae can live for up to a full year no in a vacant home with no problem. And since they’ll only hatch when there is a food source close by for them to feed upon, anyone entering a vacant home which has lots of flea pupae will be subject to an immediate attack. I suspect this is what you’re experiencing at this time.