Despite their relatively small genome, mycoplasmas can cause persistent and difficult-to-treat infections in humans and animals. A study has shown how mycoplasmas escape the immune response. Mycoplasmas ‘mask’ themselves: They use their small genome in a clever way and compensate for the loss of an enzyme that is important for this process. This could be shown for the first time in vivo, thus representing a breakthrough in the research of bacterial pathogens.