It might make more sense to scale it a bit differently:
- Up to 3 Hit Dice: roll same number of d6s, add +/- hit points to roll
- 3+1 to 4 Hit Dice: roll 3d6, ignore +/-
- 4+1 to 8 Hit Dice: roll 4d6
- 8+1 to 12 Hit Dice: roll 5d6
- Per 4 Additional HD: +1d6
- Multiple Attackers combine their hit dice
Diseases fall into tiers that resemble this revised dice progression:
- Trivial: 2d6 >= Con to infect
- Minor: 3d6 >= Con to infect or progress
- Severe: 4d6 >= Con
- Crippling: 5d6 >= Con
A Minor disease attacks either a specific body part (skin, lungs, head, muscles in limbs) or the blood or vitality (acts like being drained by a shadow.) While afflicted, the character is at Move 3, must rest twice as long every hour, and has to roll to avoid sneezing, scratching, or losing temporary control of an afflicted limb when they try to use it (5+ on 1d6.)
A Severe disease prevents movement unless the character makes a 5+ on 1d6 roll for every attempt or turn, and prevents using an afflicted body part. Crippling diseases cause permanent loss of the afflicted body part, which means death if a vital organ like the heart is the target.
Each disease description is a one-liner describing the maximum level and organ targeted, such as “severe lung disease”. All diseases appear to be Trivial when they first develop, although the GM may be rolling 4d6 or 5d6.
The GM secretly makes makes the roll on first exposure, to see if a victim catches the diseases, and every day thereafter. Each time a roll is failed, it progresses one level (from Trivial to Minor to Severe to Crippling.)
If the roll is exactly equal to the victim’s Con, the disease lingers at the current level, but if the roll is less, it improves one level. When a Trivial disease improves, the illness is over.
I've mentioned a couple modifiers to disease type before:
Fast diseases roll every hour instead of every day.
Slow diseases roll every week. Slower diseases might exist (roll every month or every year.)
Wasting diseases cause 1 hp damage per failed roll when they reach their maximum severity.
Spreading diseases spread to adjacent body parts if a failed roll includes doubles.
Written with StackEdit.