Males and females experience the burden of infection differently. Males are typically thought of as the 'sicker sex', favouring investment in costly sexual displays, at the expense of immune function. But what does this mean for the pathogen? Each sex presents a unique set of challenges that an invading organism must overcome; yet the impact of these differences on pathogen evolution has been surprisingly overlooked. The proposed project will unravel how sex-specific challenges influence the outcome of pathogenvolution. This work will show how infection in males or females can alter the evolutionary potential of disease, and will ask whether same-sex populations could ever lead to the evolution of new pathogen strains and virulence genes.