A cursory chi-square analysis of Covid-19 cases vs. what we'd expect given population sizes shows obvious geographical patterns. Though this doesn't answer *why* we see these patterns, it brings us closer to the starting line.
One hypothesis for these findings is China released a pathogen in 2017/18 that confers immunity for covid-19. A milder form of this hypothesis is that pathogen arose naturally in 2017/18 in China. ABN