Hitchhiker (species)

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