Hitchhiker (species)

The Hitchhiker was an alien symbiote that could sometimes be found inside humans. It had the appearance of a black, heart-like structure. It was believed to be harmless, and according to some, even beneficial, as it released endorphins into the bloodstream. A hitchhiker could be inside any human, and in the early 21st century one of Torchwood Three's jobs was to remove them from deceased hosts. Jack and Ianto did so to an old man, claiming that they were his neighbours. (TW: Children of Earth: Day One)