Toxic epidermal necrolysis, a severe form of Stevens-Johnson Syndrome, has about a 30% mortality rate. I understand it to be a first world problem typically caused by taking certain strong medications. It's a severe and often deadly allergic reaction.
(I'm not going to provide links because both Wikipedia entries have disturbing photos. I've verified that I'm spelling both correctly so you can google it if you want. However, you probably would be hard pressed to find an article about either condition that didn't have similarly graphic photos.)
To clarify: epidermal means it's a skin condition. Skin conditions that kill people tend to not be pretty and the symptoms are highly visible.