The bite idea is interesting, but I'm not sure it'd look good in first person. I'll keep it in mind, though - there's probably some way to make it not feel weird.
Diagnose tuberculosis as a gameplay mechanic? xD Well, I'm sure a Twilight Princess-esque smell-o-vision could work as a scan visor-style way to discover stuff you can interact with and hidden secrets.
I can see carrying diseases work as a pretty good passive ability, as well... basically, enemies you melee successfully would get poisoned and lose health over time and/or lose stats.