Pathogen adaptations during host-pathogen co-evolution can cause the host balance between immunity and immunopathology to rapidly shift. However, little is known in natural disease systems about the immunological pathways optimised through the trade-off between immunity and self-damage. The evolutionary interaction between the conjunctival bacterial infection Mycoplasma gallisepticum (MG) and its avian host, the house finch (Haemorhous mexicanus), can provide insights into such adaptations …