Hitchhiker (species)
From Tardis Wiki, the free Doctor Who reference
Hitchhikers was alien symbiotes sometimes found inside humans. A hitchhiker looked like a black, heart-like structure. It was believed harmless. Some thought it beneficial, as it released endorphins into the bloodstream. A hitchhiker might be in any human. In the early 21st century Torchwood Three removed them from deceased hosts. Jack and Ianto did so to Mr Williams, claiming they were his neighbours. (TV: Children of Earth: Day One)
