cancer
One disease to rule them all. Anything and everything causes cancer nowadays, which is somewhat depressing. There is no true cure for any form of cancer, but there just might be if the human race would stop warring with itself and concentrate on destroying diseases instead.
My God! Jimmy! Stay away from the sun! It causes CANCER! My God! Jimmy! Don't eat that! It causes CANCER! MY God! Jimmy! Don't breathe in school! IT CAUSES CANCER!