Several species of Crithidia are gut parasites of bumble bees (Bombus spp.), their main effect being to castrate the queens emerging from hibernation. The parasites are common in natural populations, spread among colonies and host species by shared use of flowers. Infection prevalence varies on a small geographic scale and over years, with no clear pattern. Genetic analyses of C. bombi show that mixed-genotype infections of single host individuals are common, associated with a high rate of sexual reproduction inside the host. Parasite populations are genotypically highly diverse, with up to 30 concurrent infections, such that the same genotype is rarely found more than once in a sample. A hallmark of this system is that a given parasite genotype infects only a certain range of host genotypic backgrounds, and this range varies among parasite genotypes. Hosts are susceptible to a limited range of parasite genotypes. A major defence Bombus uses against Crithidia is deployment of anti-microbial peptides (AMPs), which have been characterised with release of the Bombus genome. AMPs have synergistic effects. Queen mothers can also protect their offspring, the workers in the colony, by trans-generational immune priming.