Why does psorasis spread?

I had psorasis for the first 10 years on my scalp only and then 5 years ago I got a spot on my knee and later my other knee and the those got bigger and bigger over time then a couple years later on my elbows in small patches now big patches. Now its on my legs. What gives, it spreads like a fungus. Has anyone heard why this happens? ANyone else have this experience. I don't want to be old and cover in this mess. Its like a cancer just keeps spreading. Hope some of you knowlegable people got some answers. Its very puzzleing to me. Thanks! Tweet