Social species of honey bees and bumble bees carry in their gut a species-poor bacterial community that is absent from solitary bee species. To examine the role of this gut microbiota, the authors raised worker bumble bees in a semisterile environment when they emerged from the pupae, and fed them either faeces from their nest mates or sterile sugar water (controls). Following inoculation with the parasite Crithidia bombi, controls (which lacked a normal gut microbiota) showed significantly higher levels of infection and parasite numbers than the faeces-fed workers. Similarly, wild-caught bees were less likely to be infected with this parasite if they carried a normal gut microbiota. Solitary bees are known to lack this gut microbiota, so the authors propose that the social environment facilitates the transmission of commensals between nest mates during gut formation, which may then protect the bees from parasites through competition for resources or secretion of antimicrobials.