Parasite Invaders

From: Star Trek: The Next Generation

Star Trek: The Next Generation

Episode: TNG 125 - Conspiracy

A never-named race of tiny, crustacean-like intelligent beings who attempted to conquer the Federation by infiltrating Starfleet and taking over the bodies of high-ranking officers in 2364, an effort halted by Captain Jean-Luc Picard and exposed by Admiral Norah Satie. The parasites are thought to be extra-galactic.