Nanite (Stargate)

The Nanites are a sapient, parasitic artificial species based on nanotechnology. They view biological bodies as mere machines and try to take control of them.

The Nanites were created by the Ancients as a plague against their enemies, the Wraith. As such, Nanites instinctively head for Wraith tissue whenever it's available. That's how a team of researchers tried to cure Dr. Weir of a Nanite infection by injecting a sample of Wraith tissue in her body, causing all Nanites to clump in one place and making it much easier to kill them than when they were spread through her body. This worked, but it turned out a few Nanites had survived and these multiplied again, trying to take control of Weir's conscience until she was able to fight them back.