Hitchhiker (species)
From Tardis Wiki, the free Doctor Who reference
Hitchhikers were 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 2009, 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)
