Incompatibility of rhesus blood groups between Rh mother and Rh+ infant causes erythroblastosis foetalis. It is a severe haemolytic anaemia that affects newborns due to destruction of the infant's red blood cells by factors present in the mother's serum. The genes for Rh varies being rare in East Asians, South Americans and Africans, but more common in Caucasians. The survival of this gene through natural selection may be because it has an advantage value. It partially protects people against a parasitic disease called toxoplasmosis that is carried by cats but is latent in about one-third of the humans. Thus, even though Rh factor is a problem in pregnancy, it is still sustaining in the genome.