FAMILY OF BLOOD

The Family of Blood are green, gaseous sentient entities that can inhabit and possess human bodies and possess a unique sense, being able to track a scent over interstellar distances and even time period. They gather in family-based social cells or parents and children and spend their time hunting organic preys. They have very short lifespans, lasting only 3 months, but lend to their descendants technologies they've stolen from their victims. While in human form, the Family can telepathically communicate with each other from a distance. They also had access to their Hosts' memories, which enabled them fast understanding of stolen technologies but though have some difficulty assimilating personnality and pretending.

Source: Doctor Who universe (DW: Human Nature / The Family of Blood)