Last time, we looked at a variety of waterborne diseases that can make your characters' lives hell. Of course, diseases are hardly the be-all and end-all of internal problems. No, if you really want to make them feel grossed out -- if you really want to make the heroes' suffering as total as possible, give them a few internal parasites.
Simply put, there are thousands of species that live off other species. Many of them are easy to transmit, and many of those feel quite at home in the human body. You may well have a few within you right now -- bacteria, perhaps, or something you don't know about. A fungal infection, or gastro-intestinal worms -- who knows just what's crawling away inside you right this very minute?
How do we define parasites in gaming terms? In the d20 system, most parasites are of the vermin type, although in areas with high populations of magical or strange creatures, they may be magical beasts (often augmented vermin), or, if you want to get Lovecraftian, aberrations.
Who's at risk?
Parasites derive their sustenance from living creatures, and as such most of them infest humanoids, animals and magical beasts. Dragons can suffer these infestations, but often find ways of curing themselves. Conventional parasites rarely infect dragons or outsiders: they may find a way into their body when their hosts are polymorphed into more inhabitable forms but are usually killed off when the host assumes a different shape.
Undead don't often have parasites in the truest sense of the word. Being physically dead, most ill effects that affect their respiratory or digestive tracts simply do not apply, and having no Constitution scores they are able to ignore damage to that attribute. In addition, their bodies provide few nutrients to any creature unless the creature eats decaying flesh. In most cases the worst they could expect is a case of mould or saprophytic fungi. Perhaps undead parasites that feed on undead hosts could exist, feeding on the necromantic magic that animates them, leaving the host sluggish and unco-ordinated.
Aberrations are rarely prone to parasite infestation. Like undead, the alien and unnatural physiologies of these creatures provide little for the pests to live on. Only the hardiest infestations survive. As before, there may be exceptions, but they remain undiscovered -- theoretically the most successful parasites would need only the simplest substances to survive, and would be capable of replicating quickly. Consider the bacteria that wiped out the Martians in The War of the Worlds. In most cases, though, the only parasites that could infect an aberration would themselves be aberrations.