A new study in mice suggests that newborns are more susceptible to intestinal infection because they harbor undeveloped gut microbiomes, and also hints at a way to boost colonization of healthy microbes that protect from pathogens. The results suggest that an immature immune system is not the sole driver of increased susceptibility to intestinal infection in newborn mammals, as has been thought. Yun-Gi Kim et al. colonized the guts of germ-free mice with microbes from either infant mice or adult mice, before infecting them with a strain of Salmonella. About 50% of mice colonized with the microbiota of four-day-old mice succumbed to infection, they found, whereas all germ-free mice colonized with the adult microbiota survived. Mice with an immature implanted microbiota also had significantly higher pathogen loads in their feces; this effect was also seen in mice with immature microbiota that were infected with a second type of diarrhea-causing bacteria. In infant mice that were already infected with a pathogen, inoculating them with "adult" microbiota reduced disease. Further investigation revealed that a type of anaerobic, spore-forming bacteria known as Clostridia, present in adult but not infant mice, protects against the two strains of diarrhea-causing bacteria.