So...
I think most diseases should have very visible cosmetic effects on your character. You could become covered in a rash, or zits, or boils. You could ooze mucus from your nose. You're eyes could get all pink and gunky. Your skin could go all pale like you're anemic... or yellow like you have jaundice. You could get dark circles under your eyes. You could sweat profusely ... the list goes on and on. Additionally, some diseases could make you cough, or sneeze, or have a raspy wheeze with each breath. You could occasionally vomit :sick: , or have a bout of tremors.
Of course, some diseases could be much more covert and not have any visible symptoms at all.
Anyway. Just throwing the idea out there.