Variation in the MHC has been repeatedly linked to host microbiome composition (Bolnick et al., 2014; Kubinak et al., 2015; Leclaire et al., 2019; Pearce et al., 2017; Silverman et al., 2017), and this regulation by the MHC may be adaptive by protecting against autoimmunity (Silverman et al., 2017) and reducing host susceptibility to infection (Kubinak et al., 2015). The gene discussed is HLA-C; the disease is infection.