Adult fleas are stimulated by movement, and typically this movement is in reference to the host from which the fleas are feeding on (cat, dog, etc...). When a family leaves for an extended period of time, the movement stops, thusly reducing flea activity.
Upon reentering the home the "new" movement stirs up commotion and the fleas begin to feed again.....this time on you and your family. (This is also a common occurrence when a family pet passes, thusly removing the host from the home.)
So, what do you do? First, if a pet is involved, get it treated. (If you do not own a pet, the problem could be from an outside source such as a squirrel, raccoon or possum.) Next, try vacuuming infested areas to remove egg sacs and larval habitats and immediately discarding the bag afterward.
Finally, the help of a pest professional may be necessary to achieve full elimination.