What seems to happens is that newly collected fish are tanked in wholesaler's tanks. There, they are exposed to the Lymphocystis virus from previous fish housed in those systems. That begins an infection, which manifests itself a few weeks later, usually when the fish has been purchased and taken home. It has nothing to do with water quality or diet, it is all about exposure to the virus in naïve fish. Once the virus enters the fish, it causes the skin cells to grow to gigantic size, creating the lesions we all know and love. The fish then mounts an immune response, and gradually, the virus is repelled and the lesions go away. While diet and water quality have some minimal affect on the fish's immune response, the disease goes into remission regardless of those two issues - unless they are SO poor that the Lymphocystis cannot be expelled - but that is exceedingly rare. Even the worst cases seem to resolve on their own. The worst case I've ever seen was a fish from a member here, and it recovered. Here is a pic: