Wyrm wormsThese giant bowel slugs live inside dragons. They do not kill, but merely sap the dragon’s strength.
Hackmandibles. These are like fleas the size of your fist with a bite like a vice (they need it to penetrate the thick scales of their host). Enterprising but naive wizards think to use them as a source of dragons’ blood. In fact it’s harder to kill one of these than a Great Red: their exoskeleton is impenetrable to most blades and they jump up to twenty metres at a hop.
Scalefly. These are flies which colonise the epidermis behind the belly scales (the dorsal scales are too tough). They live long enough to lay hatches of maggots, which lay dormant for a few years, then continue the line. The maggots secrete juices which dissolve the scales and then they reabsorb it as food. When a scale is sufficiently rotted it will flake off. These are the main source of the infamous "weak spots" on a dragon’s belly which are penetrable to arrows.
Firetooth beetles. These are not really parasites, rather they have a symbiotic relationship with the dragon. When a dragon breathes fire it melts and burns the enamel on its teeth. The firetooth beetle eats this residue (the dragon grows more to replace it) and thus prevents the dragon’s teeth from becoming blunt and black.