We are locked in an arms race with opponents who are capable of evolving faster than we can. These microbes place a huge burden on the New Zealand health system; a staggering 1 in 4 overnight hospital admissions are infection related. So how do infectious microbes adapt to live and cause disease in their hosts? And what factors influence this arms race? We will investigate these questions using a bacterium that naturally infects laboratory mice. The knowledge gained from this study will transform the field of pathogen evolution from one which relies on anecdote and observation, to an experimental science in which the consequences of genetic change of both pathogen and host may be precisely defined and tested. Such an understanding will allow the evolution of infectious microbes to be anticipated and should highlight new 'chinks' in our opponents’ armour. In the fight against an ever-changing foe, forewarned is forearmed.