There could be epigenetic or environmental factors to blame for a disease.
But even so, assuming this is a genetic issue, proposing gene therapy in this case is putting the cart before the horse. It's like if you had a memory leak or segfault in a 3.4 billion line codebase, but the issue was only reported a few times (out of 7+ billion active users) and you can't replicate it. Obviously, a patch for this issue would be nice, but first you need to find out what's causing the issue.
But even so, assuming this is a genetic issue, proposing gene therapy in this case is putting the cart before the horse. It's like if you had a memory leak or segfault in a 3.4 billion line codebase, but the issue was only reported a few times (out of 7+ billion active users) and you can't replicate it. Obviously, a patch for this issue would be nice, but first you need to find out what's causing the issue.