Some people won't realize the importance of their health ---

not ···until意思是直到······才······时间状语引导词,这一点和汉语表达习惯不同.
有些人直到失去健康才意识到它的重要.
二者用途是不一样的
unless 除非···,如果不 ,是条件状语引导词,if ```not
I shall go tomorrow unless it rains. 　我要去,除非明天下雨
　------Shall Tom go and play football? 　　------Not unless he has finished his homework. 　
I won’t attend his birthday party unless invited (= unless I am invited)