Hitchhiker (species)
From Tardis Wiki, the free Doctor Who reference
A Hitchhiker was an alien parasite that sometimes nested in human chests. It was harmless, and according to some even beneficial, as it released Endorphins into the bloodstream. They could be inside any human, and by the early 21st century one of Torchwood 3 jobs was to remove them from deceased hosts. (TW: Children of Earth)
