I'm thinking about adding lasting injuries to my next campaign by giving some Opponents a special triggered encounter attack, that comes into effect when an attack crits, bloodies or puts a PC under 0 HP.
This hopefully ensures it's a rare event that only happens in important battles and doesn't disrupt gameplay to much, if I only give it to about 2 opponent each level (the effect still has to hit).
I didn't think about the disease track for that much, but think that's ideal for things like broken bones. I didn't use the track so far (I only played one campaign of about five sessions in 4th edition), but I'd be interested what ideas others have on using it like this.