Some thoughts so far, so the rest of you can weigh in:
Severity levels are more my idea. Mundane poisons/diseases would have a rating 1-5, that powers such as that subtract from. They would also realistically have an onset time. Most would do Bashing Damage, repeated every period of onset time. Likewise, they have a certain duration, represented by a toxicity rating. The initial Sta. + Endurance time would only slow the onset time, by a factor = to successes (usually at 8). Example:
Curare: Severity: 5, Onset: Immediate, Toxicity: Terminal
By this rating, you would suffer 5 bashing every round after being hit, till it killed you or you were cured. If you made some successes on your roll, then you would suffer 5 Bashing every couple of rounds instead of every turn.
Just haven't got all the factors figured out yet. I could opt to allow Endurance rolls to be added to Soak for a simpler mechanic, etc.
