It could mean you are not eating correctly to get the correct nutritional value to heal on. It could also mean your sugar levels are way off and they need to be checked.
It means you need to go to the doctor. Unless you already have a very low constitution(you have trouble making it to the mailbox) passing out when you're sick means you have low blood pressure or you already have something else wrong with you thats draining your stability. Go get a checkup.
If you are passing out every time that you are getting sick it could mean that your iron is low. Ask your doctor to make sure that you are not diabetic. Maybe it is your diet.