When Superman is exposed to green Kryptonite he becomes weak. It is not that it kills him, it just weakens him & makes him vulnerable, allowing his enemies the opportunity to overtake him. Everyone else is fine, it just affects Superman.
If you have an autoimmune issue or illness that your doctors have not been able to label you with yet, chances are YOU have a food that is YOUR Kryptonite. It is perfectly healthy food for other people, but for you, it weakens your body and allows you to not be at your best or even allows your enemies (illness) the opportunity to overtake YOU.