1. This was a test from specific areas in Vietnam.Ich infection rate in the wild
I read all the time people saying how infection rate in the ocean is low because the fish are healthy and have strong immune systems. Well, this study mirrors what I've read from other parts of the world (e.g. Great Barrier Reef): The Prevalence of Cryptocaryon irritans in wild marine ornamental...humble.fish
Most of us buy our fish from the LFS which comes from the wild. There is little doubt that the fish in crowded LFS tanks are riddled with parasites.
Unless you treated or observed the fish for a while, I think it’s safe to assume it carries ich.
2. significant differences were found between species - and seasonally.
I think with a low-stocking density and a resistant fish, ich will die off. However, what you're missing, is that people that rapidly stock fish - to high levels in especially a new tank, ich can be devastating - due to the more rapid multiplication.