According to Discworld medical theory as it is popularly understood, a hemogoblin (alt. spelling haemogoblin) is a microscopically tiny imp living in the bloodstream that attracts iron to itself and physically carries it about the body. Without one's full component of hemogoblins, one runs the risk of sickening with anaemia and possibly dying. (Another good reason for avoiding vampires, as they are attracted to hemogoblins).
Mustrum Ridcully, on the other hand, while not entirely dismissing the theory, has condemned it as positively unhygienic, and evidence of bad organisation and sloppy planning on the part of the Creator..