Vashta Nerada spread to many planets by unknown means. While their swarms were not common, they inspired fear of the dark in most species, since any shadow could be made of Vashta Nerada. According to the Doctor, this was why almost every species in the universe was afraid of the dark. While individual Vashta Nerada and small swarms were relatively harmless, larger swarms could strip a being of its flesh in seconds. They could also coordinate as a single being, being able to animate a space suit to move.