I need some help coming up with rules for poison that are both simple and robust enough to do what I want, which is:

• Varying levels of potency (from almost instant death to slow poison that works over the course of hours or days)
• Causes damage initially and continues to cause damage over time
• Can eventually run its course and stop damaging victim
• Possibly a mechanic (STR check?) to avoid or slow damage
• Disease will use the same system (disease is essentially a slow-acting poison)

About the system

• Uses a wound/damage track instead of hit points
• Every character can potentially take up to six [Damage Units]* before dying, but you have a chance of dying from each hit (a chance that increases with the number of hits you have already taken)
• Utilizes a "Death Spiral"; Hits result in a penalty to attribute/skill checks (important to consider if using a STR check to slow poison's damage)

Here's what I've got so far:

• All poisons cause one [Damage Unit]* initially, and one [Damage Unit]* per [time unit]   
• Weak poison causes one [Damage Unit]* per day
• Moderate poison causes one [Damage Unit]* per hour
• Strong poison causes one [Damage Unit]* per round
• Cured with magic/remedy/antidote
• Resist damage with STR Test
• Runs its course after n [time units]; (roll on 1D6 equal to or less than number of [time units] since affected?)
• Disease acts as Weak poison

I have some problems/concerns with it though:

• Seems like the "runs its course" provision is too forgiving and will keep poison from killing most people (I want it to be more lethal than that);
• At the same time, it would get harder to resist damage the more damage you take (due to the Death Spiral), which may or may not be good
• Should strong poisons run their course more quickly than weak poisons?  They would under these rules, running their course in a matter of rounds, while weak poisons would take days
• Should there even be a chance to resist damage, or for poison to run its course?  Should they have to find a cure/antidote or die?

Any thoughts?  I'm open to most ideas as long as it doesn't complicate things too much (no more than a paragraph or two tops).

Let me know if you need to know anything else about the rules.


*Haven't decided what to call these yet, I'm leaning toward "Wounds"

Well, if each Wound has a chance of instantly killing, then "runs its course" may not be too forgiving.
 
The death spiral effect could be quite nasty too. I don't think its unrealistic though. If it is a problem you could limit the effects by having Death Spiral not affect it - you could decide that the action penalty for most wounds normally represents e.g. pain penalties, which shouldn't affect checks of inherent metabolism. Or just put the poison save in some type category that isn't affected, or treat Death Spiral as a reduction to certain attributes (not including the one poison save runs off) instead of a penalty across the board. Lots of possibilities.
 
Rather than a fixed time course, you could also have a character continue taking damage until they're successfully passed N tests, or 2 tests in succession or whatever. Or, you could have successes downgrade the rate of haemorrhaging, rather than stopping the poison completely; a strong poison might do a few wounds over a few rounds, then be reduced to a moderate poison and continue killing the victim slowly.