A few days late to the conversation, but I am pretty allergic to poison ivy (Urushiol oils) and Benadryl (a practically magic antihistamine commonly used to treat it and 20 other ailments).
Oils cling to your skin, penetrate and turn into quinone. This chemical hops on white blood cells (that are there because of the foreign chemicals) and ride through the blood. Once in the blood, it can pop up any where the skin is thin, but generally close to the initial site. The more it spreads, the more white blood cells are sent, which means it can spread further. It's not contagious and not spread by scratching, but you could surely get infected if you turn a small rash into a bloody wound. I think the initial RR is just for exposure.
Once you fail that initial RR, there is a short time to use cold water surfactant soaps to wash away the oils. Wash your clothes and gear, too. That might buy you another RR, maybe with a bonus. Most people will be fine after that. A few red bumps; gone in a few days.
But once you fail your roll by enough, penalties will build up for that area. Poison ivy on your arms/hands might give you -5 or -10 on OB, drawing, Picking Locks, etc.
People scratch to remove the penalties for that area. Like an unconscious healing spell, we even do it asleep. But relief only lasts for a little while. The more you scratch, the more it itches, later.
So, after you're addicted to a substance in RM, you accrue penalties until your next fix. But that fix only lasts a while. It's like the itching/scratching associated with poison ivy.
I don't think the penalties should get too extreme.. unless it gets in your eyes, ears, groin.. Ok, the penalties could get extreme. But that could be handled by the initial exposure RR. And maybe those extreme effects could be for a serious Moving Maneuver/ Foraging/Herb Lore Fumble.
I say, roll an RR for exposure. About 85% of people are allergic. "00" your roll and you are immune!
Whatever you fail that by is the number of days it will last.
Every day it lasts, roll another RR with SD as a bonus. Fail this roll and increase the penalty to that area OR it could spread to another area (random roll, grapple chart..?).
If you want to be a mean ninny GM, have their Co bonus be a penalty. But that's just goofy-and maybe not medically accurate.Penalties accrue and it spreads until it runs it's course or you've clawed out your eyes in an attempt to reduce the penalties (because we're treating it like an addiction).
Doesn't affect animals, probably not orcs.. Probably not elves either. Never liked elves.
I have to have a steroid shot, which reduces the immune system, slowing down white blood cells, so that the quinone doesn't spread as far, so that my now weakened immune system can take care of it. But that's only been a few times. I get pretty good at noticing that stuff while hiking, now. Great health, otherwise.