American Parasite (these should not be confused with English Parasites) A parasite is very much like a hitchhiker except, instead of being carried by a letterboxer between letterboxes on its own, it is carried along with a hitchhiker. When a letterboxer joins a parasite to a hitchhiker ("infecting" it), it is stamped into the hitchhiker. The parasite's stamp is also recorded in the logbook of the letterbox that a hitchhiker is placed in, "infecting" the letterbox, as well.