The case of the O allele at the ABO locus may be an example of within-locus balancing selection: whereas the O allele clearly protects against malaria, non-O alleles may protect against other diseases caused by viruses and bacteria (Anstee 2010) which show strong comorbidity with malaria (Scott et al. 2011; Church and Maitland 2014). Here, ABO is linked to malaria.