(Rules for Creep have been adapted from Redline, by Fantasy Flight Games, and are also influenced by the Sanity rules from Call of Cthulhu.)
In the world of Ground Zero, the myriad of mutant plagues, environmental poisons, and radionucleotides have mingled to the point where, in regard to their impact on living beings, they are all the same global ailment. To the folks who roam the remains of the world, the effects of all the toxic goops and lethal pathogens are thought of as a single plague: the Creep.
In truth, the Creep can stem from nearly any source of radiation, man-made biological agents, or toxic substance. Immediate effects vary based on the substance encountered or environment braved (see Environmental Dangers), but the long term effects are the same: it's all Creep.
Creep accumulates in characters following periods of exposure to Creep-inducing agents. Although you may recover from the immediate effects of these agents, the Creep will almost certainly be with you until you die. As the Creep accumulates in your system, more radical effects emerge. In the short-term, some effects might seem beneficial (as any rad-worshipping cultist will tell you), but everyone with the Creep is ultimately heading to the same place.
Repeated exposure to the Creep leads to sickness, mutation, madness, and (hopefully) death. A character's cumulative Creep contamination is kept track of by his "Creep count", while his body's tolerance for the Creep is measured by his "Creep threshold". As the character picks up the Creep in the wastelands or blasted ruins, his Creep count rises. When it reaches his Creep threshold, his cumulative contamination level increases from "mild" to "low", and he begins to suffer permanent effects. When his Creep count reaches twice his Creep threshold his contamination level increases to "moderate", producing further effects. At even higher Creep counts it increases to "high", then "severe", and finally "lethal".
There's one important detail: the player will not know his character's Creep count. The GM keeps track of the character's count and informs the player only when his cumulative contamination increases to the next level. Also note that if a character's Constitution is drained, his Creep threshold will also decrease, leading to accelerated damage. (Temporary Constitution damage does not decrease the Creep threshold.) There is no known method to decrease a character's cumulative contamination level, even if his threshold somehow increases (though his Creep count can be decreased by using Karma Points). Medical treatments can mitigate some of the effects of exposure, but the Creep is permanent.
A character falls into an eerily still pool of silver-coloured fluid and the next morning new eyes open up on his shoulder and back. Months later, after being hit with an irradiated bullet, new fingers begin budding along his forearms. By the time he gets caught in a Rad zone and diverges into two incomplete, dying masses of tissue, he has grown another head and two new mouths.
Most Creep effects have a certain "theme" linking them. This adds an element of drama to the game that comes from a sense of impending change. Having a clue about what happens next, but not knowing for sure, is exciting.
The example Creep chain shown below (Creep Sickness) is a relatively dull example for non-mutant characters (such as stock humans). The other linked Creep chains are much more interesting.
The character does not mutate, but grows more and more sickly as his contamination increases.
Starting characters begin play with a Creep count of 1d6. For characters starting at a higher level, add 1d4 Creep for each additional level. Remember - only the GM will know the exact Creep count for your PC.
With GM permission, a character can start with more Creep in exchange for extra Build Points. A character gets 2 BPs for beginning play with a low contamination level, starting with a moderate contamination level grants 4 BPs, and high contamination grants 6 BPs. (The GM still rolls for starting Creep - which could bump your contamination level even higher.)