To add, the reason it has such a high mortality rate is TEN/SJS are almost isomorphic to 2nd and 3rd degree burns over a high percentage of your body, which is

To add, the reason it has such a high mortality rate is TEN/SJS are almost isomorphic to 2nd and 3rd degree burns over a high percentage of your body, which is also very commonly fatal. The "30%" mortality rate is greatly dependent on how bad the SJS/TEN manifests.