One very robust result of models of host-parasite co-evolution is that under regimes of mixed infection, where different strains of parasites compete for limiting host resources, parasites should evolve higher virulence strategies. This has wide reaching ramifications for optimal parasite strategies, since parasites are seldom alone in exploiting hosts. However, not all interactions between parasite strains within hosts are equal. When closely related parasite strains share hosts one might expect different evolutionary outcomes than when distantly related strains interact, particularly if the degree of relatedness between strains varies in nature such that parasites may be confronted with more or less related competitors. In addition to variation in the relatedness between interacting or competing parasite strains within hosts, the nature of the within-host interaction can vary greatly. Three types of within-host interactions have been identified, though this may not represent the whole range of the possibilities: competition for limiting host resources, production of public goods such as extra-cellular enzymes that promote infection success or efficient exploitation of the host and that can be used by all parasites within the same host, and spiteful interference between pathogen strains. Theoretical expectations differ for how virulence will change between single and mixed infections, and particularly for mixed infections of differing degree of relatedness between the parasite strains for these different types of interactions. I will discuss the outcome of experiments designed to examine how virulence differs among mixed infections that vary in degree of relatedness in the anther smut fungus Microbotryum violaceum, a Basidiomycete fungal pathogen of plants that is transmitted by insect vectors, with a small aside to examine the idea of relatedness and the applications of kin selection to microbial interactions. Individual strains, when involved in mixed infections, drew greater resources from their host plant and produced more spores per flower than did strains that infected on their own. Furthermore virulence, this time measured in terms of the degree of sterilisation of the host plant, varied among mixed infections that differed in genetic relatedness among strains. Individual strains interfered with each other's ability to colonise the plant, thereby reducing the virulence of mixed infections for this trait, but interfered less when strains were closely related, suggesting spiteful interactions. Thus for different traits of the infection genetic relatedness among interacting parasite strains differentially influence virulence.