Associations are increasingly recognized between parasitic protozoa and smaller organisms such as bacteria (see Parasitology Today 1, 143), and viruses. Their revelation, originally by ultrastructural studies, may now be indicated by observations of 'rogue bands' on gel electrophoresis of parasite nucleic acids. The origins and roles of parasite 'parasites' are still uncertain, but, as Michael Miles suggests here, they may prove of particular importance as agents to introduce genes for important immunogens into non-pathogenic protozoa for vaccine delivery.