I blame a lot on the petstores. It just seems like keeping one in crappy conditions while it's very young could lead to problems that keep coming up later on, especially if they don't get the right vitamins or get seriously malnourished.
When my w/c or breeder-bought animals get sick, it's like I treat them and they get better, and there are no more worries. With my petstore animals (including my pac) it seems like there's constantly some sort of minor health issue going on.