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 a

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 https://academic.oup.com/jid/article/208/8/1286/2192838