Unfortunately Genetic "weakness" is becoming quite common in New Zealand these days.
Damage caused by the environment, is irrelevant, as it has little to do with the disease itself and is also subject from case to case.
Separating out, is often only done when strain breeding, and it is genetic "weakness" that has caused the problem of the females birth canals being too small or stressful to release there fry in the first place. Often as not, supplying a tank planted out with few males is good enough.
Water parameters are again subject from case to case, and have little to do with the actual disease that is causing the mass of death and deformities all around New Zealand
Stress describes most of the above, and is a very general issue, faced by all keepers of all fish, not just guppies, and is commonly rectified easily.
Shae