I do not believe the incidents of the more common forms of ich, such as vulgaris or X-linked are evenly distributed through all population groups. The extremely rare and severe forms of ich may pop up in any population group because of genetic mutation. But that is not the case with Vulgaris.
Vulgaris is a lot more prevalent in subtropical and tropical climates because the symptoms are less severe in those climates and people who suffer from it can pass the genes on through the generations. On the other hand, someone with severe vulgaris and a sweat disorder would have a lot of trouble surviving, much less procreating, if he is from the northern latitudes and from a cold and inhospitable climate. I would say in centuries past infants with severe ich of any kind may be exposed by their families.
In some coastal provinces of southern China Vulgaris is prevalent to an extent that is unimaginable in the West. I would not be surprised if it originated centuries ago in China.
If you are American and live in the States, demographic statistics may say that probability that you are of northern European descent would be high. However, I do not think so.