It is quite difficult to get ride of fleas from the house. Different from what most people think, fleas do not live permanantly on the animal but on the floor, on carpets, pillows, etc...

Fleas generate eggs. From the eggs, a larva follows different development steps to turn into a nymph and to finally become an adult insect. Nymph are inside a cocoon and they have the particularity to stay "asleep" a very long period of time and to turn into an adult insect after some time that can last for several months. It is the reason why we can see fleas appear in a house left empty for several months in the absence of animals.
In that case, these nymphs can wake-up, when they start feeling vibrations on the floor.

The adult insect has to bite a target (a dog, a cat or an human) in order to feed himself with blood. After feeding, the flea is falling on the floor where it lives and deliver its eggs. And so on....

Flease and larva are affected by most of the common insecticides: insecticides used on animals (collars, powder,...) or household environment insecticides.

However, the eggs are not affected by those and nymphs are quite resisitant to those.

So an effective treatment consists in breaking the flea cycle and to prevent the fleas to deliver new eggs.