Two subpopulations can be identified within populations of Salmonella enterica subsp. enterica serovar Typhimurium: a slow-growing one that promotes inflammation to outcompete the gut microbiota through expression of type III secretion system 1 (T3SS1); and a fast-growing one that does not express T3SS1 and is phenotypically avirulent. Here, the authors observed that, in addition, genetically avirulent mutants that do not contribute to inflammation ('defectors') arise during infection. Importantly, the defector subpopulation increased in size more rapidly when the phenotypically avirulent subpopulation was absent, leading to reduced inflammation and the decline of the whole S. Typhimurium population in the gut. This indicates that the phenotypically avirulent subpopulation protects the slow-growing virulent subpopulation from being taken over by defectors. Thus, S. Typhimurium subpopulations exhibit cooperative behaviour, in this case through bistable expression of virulence genes to stabilize virulence.