HIV sequences circulating in a human population will harbor adaptations that reflect the HLA alleles expressed in that population. Panel A. In a population with high HLA diversity (shown by different host colors), the risk of acquiring an HIV strain pre-adapted to host HLA (i.e. the risk of acquiring a virus whose color matches that of the host) is relatively low. In such a setting, population-level HIV adaptation to HLA will occur relatively slowly (indicated by minimal shifts in virus color composition over time). Panel B. In a population with low HLA diversity, the risk of being infected with HIV pre-adapted to host HLA is far higher. Furthermore, over time, HIV strains harboring adaptations specific for common HLA alleles in the population (shown here in yellow) will increase in frequency more rapidly.