这不奇怪。这句话原本是：
1. One realizes the importance of health only when one is ill.
现在，有两种强调方式：
第一种，把only when one is ill 提到句首。
第二种，用It is强调only when one is ill。
2.  Only when one is ill does one realize the importance of health.
【注】在句首的是only，符合主句倒装条件。
3.  It is only when one is ill that one realizes the importance of health.
【注】在句首的是It，而不是only，无法满足倒装条件。
这跟 not... until...是一回事。例如：
1. He didn’t realize his mistake until the teacher had told him about it.
2. Not until the teacher had told him about it did he realize his mistake.
3. It wasn’t until the teacher had told him about it that he realized his mistake.
以上两组句子，只有句2 能够满足倒装的条件：only和not在句首。
若有不足或错误，请黎老师及各位批评指正。