In Rokugan, the land of the books and games Legend of the Five Rings, there exists a place called "The Shadowlands". (See The Shadowlands Horde.) This place is a literal "Hell on Earth" with an openly hostile and aggressive landscape (the ground will swallow you whole in some places), mutated animals, horrible oni (demons), and all sorts of other nastiness.
That area wasn't always such. When the dark god Fu Leng fell to the earth from the heavens, he landed hard enough to open a pit (The Festering Pit of Fu Leng), and he unleashed forces that changed the landscape and the flora and fauna of the region. The former residents, the Nezumi (or Ratlings) were immune to this change, but everything else mutated.
The other effect of the taint is that it can be transmitted from one person to the next. The taint is somewhat of a combination of our fears of radioactivity mixed with our fears of disease.
Like syphilis it causes your mind to break, you eventually become ruled by the taint, and turn to the "dark side" of the Shadowlands. It can be transmitted through personal contact, even through mere proximity, so once you contract the Shadowlands Taint, you're treated like a leper. Like leprousy, your body will start to change, and usually not in "Teenage Mutant Ninja Turtles" benign fashion, but you'll start to lose your skin, your eyes will fall out, or you'll sprout useless and hostile limbs. Your own body could start attacking you. Sometimes you'll gain "beneficial" changes, like claws, or wings, or extreme strength. And it won't just transmit from person to person, The Shadowlands can infect a person just as easily as being clawed by a Shadowlands creature can.
The taint also affects the dead as readily as it affects the living. A corpse left in the shadowlands will rise as a zombie or a skeleton, leaving The Crab Clan who defend Rokugan from The Shadowlands Horde with the unpleasant task of having to face their own family after they've fallen. To be sure that the bodies don't rise again, the Crab often behead corpses that have fallen in The Shadowlands. In addition, Rokugani burn their corpses.
Once you have the taint, there is no real cure. The Naga have a method of cleansing the taint, but most Rokugani do not associate with the Naga. (In fact, during the era of The Four Winds, most of the Naga have gone into hibernation.) There is also a ritual where several shugenja can give up their lives to take your taint from you, but give it to themselves. Seeing as several people have to sacrifice to save one, this is an unusual ritual. Most tainted individuals either keep silent (until they are discovered by a tsukai-tsugai, or witch hunter) or they join The Damned, a group of individuals who have fight in the Shadowlands since they have nothing to lose, and hope to die before the Taint takes over their mind.
The taint is a very intriguing development in a game-world, since it gives a very easily identifiable "Evil" to the world. Where "Detect Evil" always somewhat bothered me in Dungeons and Dragons (I'm Chaotic Evil, it says so on my character sheet), the idea of "Detect Taint" and a whole background to it seems much more satisfying. (The other axis on the alignment chart, "Lawful/Chaotic" is replaced with "Honor/Dishonorable.")
In a land where death is not a fear, the Shadowlands Taint represents the worst fear possible.