A skin condition that looks like and makes you feel like shit.
Psoriasis is an autoimmune disorder caused by overactive T-cells. It makes afflicted areas of skin red and covered with white scales. It's usually most common on the elbows, scalp, knees, and anywhere where there is more skin turnover (when skin cells are dying and being regenerated more than usual).
There are variations on psoriasis, including guttate psoriasis, which is more widespread and appears sort of like random spots of grossness. There's also versions that make your skin appear burned, or even in pus-filled pustules that make psoriasis suck even more.
Passerby: Dude..... What happened to your arm?
Poor Psoriasis-afflicted person: Nothing. It's psoriasis.
Passerby: Oh, I'm sorry... Wait... What's psoriasis?
Poor Psoriasis-afflicted person: Like cancer on the outside of your body, only it won't kill you.