In the field, pathogens exist in a diverse and dynamic ecological environment with other organisms (Blakeman, 1993; Preece & Dickinson, 1971). We tend to assume that these other organisms are ‘benign’ or perhaps in competition with the pathogen to occupy a particular niche. However, bacteria are often observed to be in close association with pathogens, as witnessed by the difficulty of eliminating them during isolation of pathogens involved in plant diseases. We now have clear evidence (Dewey et al., pp. 489–497 in this issue) that they are sometimes neither benign, nor in competition, but actually helping the pathogen successfully infect its host even when the bacteria themselves are not pathogens of that host.