Fleas can make life unbearable for both humans and pets when brought in to a home by the reaction to their biting.
Laying their eggs in soft furnishing and carpets the larvae feed on the dirts of adult fleas which contains a mix of digested and undigested blood.
Fleas can go into a state of almost suspended animation if food is short. It is quite often people moving into homes after the previous occupiers have left some time before won't get bitten immediately. After a few minutes or hours the fleas, sensing vibrations from their new hosts, emerge from their diapause and start biting with a frenzy.
Vacuuming helps dramatically to remove these pests as this can reduce the flea numbers by up to 90%.
Another good tip is to have a lit candle in a shallow bowl half filled with water (to prevent fire). The heat of the flame attracts the fleas who jump at the flame and are caught in the water.
When changing the water as the fleas inevitably won't drown make sure water is thrown away outside and the bowl rinsed to prevent you bringing the pest back.
Treatment is by a good quality residual insecticide which may need to be repeated. Once in their pupal case fleas are safe from most things except a good vacuum.