The non-carriers are not a control group unless we are 100% sure that the assignment is random. I find it hard to believe when the vast majority of population are carriers.
It's not random. More developed countries and richer people in more developped countries have a lower incidence rate. The chance of catching it increases with age. The part of the population that doesn't catch it seems to be avoiding it by luck, higher hygiene, and fewer vectors. See: https://academic.oup.com/jid/article/208/8/1286/2192838
