#4 would be interesting to me. I have a patch of Psoriasis on both legs. I used to have it in my scalp but that has pretty much gone away on it's own. The medications prescribed for it are pretty useless.
It starts when you get a scrape or injury to the skin, it never heals.
It's not contagious though, it's a genetic failure that basically shows up when you get older. It's actually related to skin cancer where the skin cells reproduce abnormally and cause irritation but isn't the same as ... it isn't a situation that could become fatal. It does crash your self esteem though.