You're not making a serious effort to guess at what difference it might make, though. Example (this is actually true!) -- I have a life-long inflammation in bo

You're not making a serious effort to guess at what difference it might make, though. Example (this is actually true!) -- I have a life-long inflammation in both eyes that started in one eye when I was about 5, and eventually spread to the other. The specialists weren't able to figure out the original cause; their best guess was basically a speck of dust -- just the wrong speck of dust (maybe with a microorganism in it?), in the wrong place, that my immune system attacked and then just kept on attacking even after the original speck had been gone for years. Medication plus a series of unpleasant surgeries have managed to keep my eyes relatively functional, so far, but others with the same condition certainly do lose all vision. Obviously this isn't a common result; but given enough specks of dust.... Well, you have to start doing some math.