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 a old man, claiming that they were his neighbours. (TW: Children of Earth: Day One)