more like worst achievement ever!
ok so this is how this works
get contagion (mage 4th level) and disease CHARNAME with your OWN party member (!). this is the only way to get a proper diseased status instead of nauseated (which didn't exist in vanilla as far as I remember, only diseased existed). btw contagion is ONLY available in sorcerous sundries in BG east.
poison via divine spell poison (4th level) or run into a spider (there are some randomly in durlag's tower for example)
fatigue easiest via a few haste spells (mage 3rd)
and yes, it only works on the main character. and it will take many many tries to apply diseased likely. time spent on this shit before I got it to work: 90 minutes!