Eusocial queen polyandry increases genetic diversity within colonies, and genetically diverse colonies presumably suffer less from omnipresent parasites than genetically homogeneous colonies. So eusocial queen polyandry may have evolved in response to parasite load. Ewald’s theory of the evolution of virulence specifies conditions favoring the evolution of virulent, damaging parasites. Applying these ideas to parasites of ants, it is inferred that parasites are more virulent, and hence queen-polyandry is more often expected, when colonies are larger, when colonies are founded dependently, and when colonies are mobile or nomadic. An analysis of an ant-database supports these predictions. Further analyses, preferably using a generally accepted database, seem valuable.