Pathogenic bacteria are highly specialized micro-organisms that are a leading cause of human, livestock and crop disease, resulting in significant health and economic loss worldwide. How these pathogens, which sometimes even live inside the cells of their hosts, once evolved is poorly understood. An international team of researchers has now found how bacterial pathogens evolved. Their work, published in Nature Microbiology today, reveals that the transition from free-living ancestors to modern day pathogens progressed in a stepwise manner—involving gene gain and loss events, but also the repurposing of existing genes.