I was reminded about this from an unfortunate incident - someone discovering a bite from some unknown critter. Annoying. Potentially harmful or even lethal, if it's the wrong thing.
Lots of this stuff comes up in real life:
- surprise bug bite!
- where did I get this cut?
- I must have gotten scratched!
- I guess I did touch my eyes after I touched that infected person.
- Oh, that was poison ivy vine? It just looked like a bump on the wood in the dim light.
And so on.
I've usually had entries like these on my Wandering Monster tables. I joke about Wandering Damage but it's a very useful concept. Just divorce it from the joke of "skip the monsters, just apply the damage they'd do." Sometimes, harm just happens.
So I put things on my wandering monster tables like:
3: Infected critter bite! 1 HP damage (ignore armor/clothing DR) plus HT roll vs. disease (-1 to all stats until cured).
5-6: Bug bite for 1d-4 FP.
1: Poisoned! Possibly a bite, a cut from an envenomed thorn, etc. Ignores DR, roll vs. HT - 1d or take 1d toxic damage and -2 DX until all damage is cured.
I've thrown on disease rolls (HT roll), colds (HT roll), unstable ground (DX roll!), traps (Per roll), rainfall (HT-based Survival vs. FP loss, DX-based Cartography to shield the map, etc.), weird smokes or gases (almost anything to roll against, sometimes nothing to roll against), etc. to represent a hostile environment.
Why I like this:
Lots of PCs get pretty blasé about combat. It's something their characters do well by design, often by basic game design (for example, a fighter in D&D-based games or knights in DF.) Unless a monster is tough enough to be a legitimate threat, a combat encounter probably just costs time and may not even cost significant (or any) resources.
Plus, combat is often avoidable. Run from the slow monsters. Hide from the one with poor senses. Talk to the one who doesn't want to die just because it ran into you on the way to the toilet.
But wandering damage of this kind isn't. It just happens unless you took precautions, such as anti-bug lotions, sealed armor, certain spells (-1 to all spells up per incidence of them), etc.
All the armor in the world won't help you against a curious spider or annoyed ant if it's unsealed. Even if it is, unless you're breathing air through a filter you can still suck in something harmful. "I have plate armor!" shouldn't mean "So I'm immune to anything that causes less damage than I have DR." Sometimes the guy with scratchin' holes in his ratty shirt or who won't wear heavy armor is on to something - in certain places the real threats are the brown recluse who drops down on your neck during break, not the sword blow you fear you won't parry.
And this kind of stuff just happens in normal environments. In hostile ones, during travel, they're bound to happen more. They make the game feel more realistic, too - how come you fight giant ants but no one ever just gets bit by one?
Really, not much to say beyond that. "Your knight was bit by mosquitoes!" is pretty funny. Even dragon-slaying heroes step on the wrong spot and twist an ankle or get bit by a bug.
Resolution is quick. Roll, roll, the PC rolls, you describe, stuff happens, you keep moving. It doesn't take a lot of time to resolve. So you get a good bang-for-your-buck time wise.