The influence of changes in population structures of modern humans and human pathogens is likely reciprocal. In my opinion, a quantitative approach to study this co-influence in a historical perspective requires, in particular, adequate estimators of genetic distances that are well developed for human but not yet for microbial populations. Here, I propose a simple measure of genetic distance between geographic populations within a microbial species based on the observed difference in the frequencies of its genotypes. Further, I apply the proposed method to principal components analysis of Mycobacterium tuberculosis, and interpret the geographic distribution of its VNTR haplotypes in the light of human historical and recent migrations. The proposed approach may be helpful for a quantitative understanding of human-microbial interactions that constitute an integral part of the global history.