It was an unintended side effect of attempting to use gene augmentation developed for humans, causing a mutation and disease to spread. They developed a cure wi

It was an unintended side effect of attempting to use gene augmentation developed for humans, causing a mutation and disease to spread. They developed a cure with help from the crew of the enterprise/phlox but that cure didn't immediately fix the mutations, it took generations. That's why I called it an accident.