Of course not vampirism, I mean something like it. Something you catch in the wasteland from a deadly creature...if you survive the battle.
I'm thinking something like your physical appearance changing. The first few days your character's stats lower, your character becomes pale and loses some weight.
Then, after a few day if you aren't cured(if say you were cured then you could never catch it again), your stats start to come back up again, you start to come back to your normal weight. Your muscles start to grow, your character looks very pale with dark circles around the eyes. Your eyes go very bloodshot.
After another few days your character keeps becoming more poweful, and your appearance shows this. The skin gets to a stage where it is almost white, and your body is creased and scarred(not like an old guy, but more evil cool character). When you get to the extreme stages of this illness, some of your bones come out of the sides of your cheeks, you become more like a walking dead, but not all flakely like a ghoul, and radiation wouldn't heal you it would damage you 2x-5x more.