OK, I have a pretty general question.
Here's the scenario: my sister has this disease in her knees, which is basically when the bones grow faster than the tendons/ligaments whatever do. so she has been out of commision for sports for a few months now (she had to quit soccer and hockey). a few days ago she fell off my sk8board onto our friends' lawn and sprained her wrist, or possibly broke it. she is constantly getting injured or hurt or something, even thought she can't do any sports.
I am doing fencing, obviosly, and am always doing crazy things like wading barefoot in swampy lakes and therefore impaling myself on branches, or playing football or slide-tackle soccer or diving for bases in kickball, etc. yet i almost never seem to get hurt.

she does practically none of this because of her knees.
what is up with that? is there some sort of reason for her getting the lions share of injuries, or am i just wicked lucky and one day will just break a leg or something for no apparent reason?
*same deal with immune systems. i'm the only one in our family who hasn't been sick in at least 3 years. but that's another story...*