Vashta Nerada were swarming, carnivorous beings. The name meant "the Shadows that melt the flesh".
The Vashta Nerada were microscopic beings that lived in swarms, thousands strong. On their own, Vashta Nerada were not a threat. They were very small; the Doctor once claimed that the dust specks visible in bright sunlight were single Vashta Nerada or a small swarm not yet large enough to be a threat to most life. Once they were in sufficient numbers, they were able to strip a creature to its bare bones in milliseconds and the the Doctor once described them as "piranhas of the air". Vashta Nerada swarms were sentient and two spacesuit-inhabiting swarms developed the ability to communicate with the Doctor through the spacesuits' data chips. They were also able to coordinate their efforts and make the spacesuits walk, though only with some difficulty, and could walk slowly in a 'zombie-like' fashion.
The Vashta Nerada lived in the darkness and cast shadows when they entered the light. On its own, outside of the darkness, a Vashta Nerada swarm would look like a shadow cast by nothing. They would mimic the shadows of their prey in order to get close, which meant staying in the light was the only option for escape. If someone had an extra shadow, it was already too late for them. Sonic technology is able to detect the difference between a Vashta Nerada swarm and an ordinary shadow. The Doctor believed they had no weaknesses (other than the light) and that the best thing to do was to run.
The Vashta Nerada lived on almost every planet which had organisms with meat, including Earth. They were born as microscopic spores in trees and lived their lives in forests. These spores were resilient enough to hatch if the tree was processed, even if the wood was converted into paper and into books, which explained why so many were present on the planetary Library. It was said that they were able to hatch and form into a swarm in minutes.
Vashta Nerada had existed since the early universe, and spread to many planets across the universe. While these swarms were not common, they inspired a fear of the dark in most species across the cosmos, since any shadow could be made of Vashta Nerada. They were rare, and on some planets, such as Earth, they survived by scavenging meat from road kill.
In the 50th century one planet suffered a large infestation. A library planet, called 'The Library', was attacked by over a "million million" (1,000,000,000,000) Vashta Nerada, due to the spores inside the books' paper hatching as well as from the wood the library was largely made up of. The 4,022 people on the planet were unsuccessfully evacuated by the Library's computer and the planet was empty for 100 years until an archaeological expedition and The Doctor landed on the planet.
Though much of the archaeological team was eaten by the Vashta Nerada, the Doctor was able to convince them to stop and give him a chance to return the people stored in the Library's computer. They agreed to give him a day upon reading about him in the library, in which time the humans were rescued and left the planet to the Vashta Nerada.