The ABO blood group was one of the first genetic markers used in genetic anthropology, and the worldwide distribution of the various alleles was one of the first known for any locus. It also has a long history in medical genetics as a marker, and there are myriad associations of blood type to various diseases (many of these are probably spurious; see the introduction to the second article).
With that in mind, it was interesting to see this (properly done) study showing an association between type O blood type and reduced severity of malaria:
Malaria has been a major selective force on the human population, and several erythrocyte polymorphisms have evolved that confer resistance to severe malaria. Plasmodium falciparum rosetting, a parasite virulence phenotype associated with severe malaria, is reduced in blood group O erythrocytes compared with groups A, B, and AB, but the contribution of the ABO blood group system to protection against severe malaria has received little attention. We hypothesized that blood group O may confer resistance to severe falciparum malaria through the mechanism of reduced rosetting. In a matched case-control study of 567 Malian children, we found that group O was present in only 21% of severe malaria cases compared with 44-45% of uncomplicated malaria controls and healthy controls…This work provides insights into malaria pathogenesis and suggests that the selective pressure imposed by malaria may contribute to the variable global distribution of ABO blood groups in the human population.