I just add a level of exhaustion each time someone drops to zero HP (and presumably doesn't die). Fluff descriptions about what injury they have (broken arm, sprained ankle etc.). Any of the tables posted here are suitable.
this, with a twist.
I like the various tables for injury here, but stacking exhaustion is just much cleaner and easier.
max exhaustion levels: 10, dead it you would get 11th
-1 to all d20 rolls, -1 AC, -1 all DCs, per exhaustion level
-5ft speed/ 2 exhaustion levels, round down.
every time you drop to 0 HP you get exhaustion level.
this levels can be removed by getting healed to 50/75/100% max HP(depending on how gritty you want it) or after Short or Long rest, again depending on how gritty you want to have it.
every 24hrs without sleep: 1 exhaustion level(this is removed by 1 level per every Long rest)
every 12hrs without water: 1 exhaustion level
every 2 days without food: 1 exhaustion level
use it for most poisons and diseases and even with variant old age penalties.
variant:
middle age: 1 permanent exhaustion level
old age: 2 permanent exhaustion levels, replaces middle age penalty
venerable: 3 permanent exhaustion levels, replaces old age penalty