Diseases
Diseases are pathogenic Maladies that are transferred from one creature to another (or from an infected area or item to a creature). A creature can become exposed to a disease in a variety of ways, such as proximity to an infected individual, contact with an infected individual, or through a wound such as a bite or claw from the infected. Regardless of the method (which is dependent on the particular disease), once a creature has become exposed it is at risk of being infected by the disease and becoming symptomatic.
Malady
Each disease currently afflicting a creature counts as a separate Malady.
Pestilence
Pestilence is a measure of how dangerous the disease currently is. When a creature contracts a disease, its current level of Pestilence determines the symptoms (effects). Through the course of play, its level of pestilence can change, but the effects of disease based on pestilence only change after an extended rest (although, certain effects can force a change in symptoms at other times). Pestilence applies equally to all diseases a creature may have (don't track pestilence separately for each disease).
Contraction
When a creature is exposed to a disease, it must make a Resilience check against the exposure DC. This exposure check is penalized by the current number of maladies affecting the creature (-10 per malady). Succeeding the check means that the creature avoids contracting the disease (but may be exposed again in the future). A failure indicates that the creature has contracted the disease, and the creature also gains the Points of Pestilence specified.
Special: If the creature already has the disease, it does not make an exposure check and instead automatically gains the specified Points of Pestilence.
Natural 20
A creature that rolls a Natural 20 on an exposure check does not contract the disease, regardless of the normal result.
Valen the rogue touches a fungus covered corpse while looting it and causes a cloud of spores to plume over him. The spore cloud causes 1 point of Pestilence (Spore Fever, Exposure DC 15). Valen must make a Resilience check versus the DC of 15. Unfortunately, Valen already has a moderate injury (a Malady) and has a -10 penalty to the check. The check fails and Valen contracts the disease, suffering 1 point of Pestilence.
A short while later, Valen is spit on by a Spore Zombie, which causes 1d3 Pestilence (Spore Fever, Exposure DC 10). He already has contracted Spore Fever, so he doesn't make the exposure check and instead automatically gains 1d3 Pestilence. The result of the 1d3 is a 2, and so Valen gain 2 more points of Pestilence, increasing his total to 3 points.Progression
Once a creature contracts a disease, it must check the disease's progression at the end of every Extended Rest. Roll a Resilience Check with a DC determined by the specific disease. On a success, reduce the Pestilence level of the creature by 1. An additional point is removed for every 5 that the DC is exceeded (i.e. a roll of 22 removes two points for a DC 15 disease). On a failure, the creature gains an additional point of Pestilence.
Multiple Diseases: A creature with multiple diseases must check against each disease and take the worst result from all the checks. This means that a creature with multiple diseases can have a much harder time recovering.
After making the check, compare the creature's current level of Pestilence to the Specific disease to determine the effects of disease for the day. These effects persist (even if the Pestilence level changes) until the creature takes another extended rest and checks the disease's progression again. Some powerful abilities may change the current effects of a disease sooner than the next extended rest.
Valen, who contracted Spore Fever and has a 3 points of Pestilence, finishes an extended rest and must check the disease's progression. He makes a Resilience check (DC 12) and fails, which increases his Pestilence level by 1. He now has 4 level of Pestilence, which DC of the disease in now increased to 14 (by the Interval Factor of +2). At this DC, the creature is still in Stage 1 of the disease and does not suffer further symptoms.
After another 24 hrs, the target fails another Resilience check (DC 14) and the DC of the disease increases to 16. The creature now progresses to Stage 2 where its flesh begins to rot and it becomes contagious to other creatures (touch exposure).
After another 24hrs, the target is required to make another Resilience check (DC 16). If the target fails this check, it will die and become a Spore Zombie. If the target succeeds, it will finally be cured of the disease and its symptoms will cease.