If it were me, I'd do a "Poison and Disease" mod, which systematically covers the poison and disease effects of all creatures in the game. My (main) request for such a feature would be to do a standardized disease system that other modders can use.
* After an incubation time (a few game hours?) the effects of the disease start kicking in, e.g. -2 CHA every turn for n turns for regular mummy rot. The disease in non-cumulative.
* Each hit from the creature imposes a one-time disease effect. E.g. each hit from a mummy lowers CHA by 2. The disease is cumulative (obviously).
* Much like PnP but with shorter delays, you experience the effects of the disease at regular intervals, e.g. -2 CHA every hour for regular mummy rot. The disease is non-cumulative.
If it were me, I'd use combinations of 1 and 2 depending on how powerful the creature is. What would make disease especially dangerous (compared to poison) is that it can spread from one character to another. Now that opcode 272 is fixed, use that with a nested opcode 146 (cast a shell spell).
Ideally I'd like to differentiate diseases from poisons (with poisons having immediate effects) and make them have reversibly permanent effects (i.e. the effects last until cured).
I have a working
AD&D poison system archived somewhere with onset times and everything, with slow poison slowing delaying the onset time.
IMO the main difference between disease and poison would be the following:
- Poisons would have onset times, after which they do a fixed amount of damage. Disease takes longer to kill one person. All diseases (if left untreated) spread by monsters should be deadly within one week. Not all poisons are deadly, but the ones that are kill very quickly.
- Diseases can (in most cases) spread from one person to another. This would not be true for poisons.
[1] Some PnP diseases can only be healed by some more exotic spell, like Regeneration or Heal. But due to the technicalities involved, and to avoid confusion, I'm reluctant to implement this particular aspect of those diseases.
This is easily doable by adding sectypes and using timing mode 9. I haven't tried this with the disease opcode, but it works for pretty much everything - SpellPack B6 prevents characters killed from Finger of Death from being raised by giving raise dead / resurrection special sectypes. These sectypes are protected against when a character is hit by a finger of death/pwkill using nested eff files (General of "Dead" is automatically set by the engine for dead characters) with a timing mode 9.
The effects that protect against the raise dead sectypes have their own sectype in a shell spell called "Permanent Death". This sectype is (successfully) removed when a character is raised by a wish or reincarnation, so characters killed again by normal means can be raised/resurrected.
-Galactygon