We all know parasites: vile, squirmy intruders that hitchhike on unwitting hosts. Or do we? A new look at these creatures in NATIONAL GEOGRAPHIC‘s October 1997 article reveals that they are both more complex and less passive than ever imagined. Indeed, parasites may be key actors in the evolutionary drama. See for yourself. Follow the odyssey of the thorny-headed worm or try staring down the inhabitants of our portrait gallery.