Your average fantasy rpg is set in a medieval world, which means knights and dragons and disease. Knights and dragons are easy enough to stat up, or I suppose they are since every game has them in one form or another. Disease, on the other hand, can present a few problems. If disease is going to play a roll in the game, it needs to be a real obstacle. If we're being realistic, we know that many diseases, if contracted, must have the power to kill or really screw up a PC. That's problem number one - explaining to a player why the character he has lovingly nursed through countless acts of daring to a lofty level is now dying from some pox he picked up when he was foolish enough to enter a town to buy supplies and train. It's a real anti-climax and seems either terribly random or terribly unfair - a couple rolls of the dice, and microscopic entities that the locals haven't even discovered have just accomplished what the Dark Lord and all his minions could not. Problem number two, of course, is that none of this will actually happen, because the chance that the afflicted cannot find a cleric to cast cure disease (or remove disease, depending on your edition) is slight. So, you go to the trouble of introducing the black plague, the disease that ravaged Europe and and maybe changed the course of human history, and the players see it as a mere inconvenience - slightly less annoying than death, but nothing that can't be handled. To me, this just won't do.
When designing my campaign, I wanted disease to be represented and I wanted it to be a problem. I looked at many different disease systems, from Arneson's in Supplement II to Gygax's in the old DMG and the ability score damage in 3rd edition, and none of them solved the aforementioned problems for me. And then, I started thinking allegorically.
I don't run a historically realistic campaign. Nod is a world of folklore, fairy tales, mythology and superstition. The medieval mind did not see disease for what it was. Rather, it imagined that disease was a punishment from God. The Black Plague was God's judgment on mankind. It was one of the most morally, spiritually and psychologically damaging event in European history, right up there with the First World War and its trench warfare and chemical weapons (which were a major inspiration for Tolkien's Mordor.) This "psychic damage" is quite apparent in Peter Brueghel's Triumph of Death (a detail of which can be seen above). And that painting got me thinking. A disease is terrible on a personal scale because it scars, weakens and kills. But disease is terrible on a grand scale because it infects and spreads. What monsters in the game we all love infect and spread? The undead, of course - or at least some undead. That's when I decided to embrace the medieval and ancient allegories (symbols) that fantasy role-playing turns into creatures and makes real with stats. So, those disease rules that I could never quite get right were out, and plagues of undead took their place. The Black Death in Nod would not be an outbreak of bubonic plague transmitted by fleas and rats, but rather a terrible judgment from Heaven by which the dead rose from their graves and spread devastation and madness throughout the land. This was something that players could deal with - opponents to be overcome and mysteries to be solved (why are the gods angry? how can we placate them?) - with their skill at the game rather than a couple arbitrary dice rolls. Of course, mummy rot and lycanthropy were still in - you can't have a proper campaign without mummy rot and lycanthropy. But otherwise, the undead, especially those who can spawn with a touch of their spectral hands, would take the place of disease in my campaigns.