According to Encyclopædia Britannica, dog fleas can live on humans or other household animals if their preferred hosts are not available. As humans lack body hair comparable to dog and cat hair, fleas find it difficult to stay attached.
Furthermore, Encyclopædia Britannica states that the compressed body shape of fleas make them suited to navigating fur and feathers, thus avoiding detection. Their spines also allow them to cling better to the host. As parasitic animals, fleas have specialized mouth parts designed to puncture flesh and draw blood. In some instances, these mouth parts allow the flea to remain better attached to the host.