A morbidly fascinating springboard to some very basic ideas about evolution, parasitism can be a marvelous teaching tool. A few of the mechanisms by which some organisms have evolved to make their living at others' expense are presented here. Plasmodium, the causative agent of malaria, is shown infecting red blood cells. The fungus Puccinia, the cause of wheat rust, is seen on a cross section of its host plant. Trichinella, the agent of trichinosis, is examined encysted in mammalian muscle tissue. And the human louse, Pediculus humanis, is viewed in its entirety-grasping appendages poised.