Best way to do it, IMO, would be to set up a global script (AKA put the script on a triggerzone or dynamic light.... triggerzone is my prefered choice) that uses an entity timer and subtracts health from the player every couple seconds or so. You'll probably want to attach that to a variable to trigger the poison effect.
Same goes for enemis, except they will need to have the poison effect put directly into their scripts.
Hope that helps you out.
The one and only,
Only those who sow the seeds of their desires will reap their benefits later.
However, I have seeds of my own to tend to. I don't have time to be someone else's watering can.