The extensive variation in the amount of harm that pathogens cause their hosts has intrigued biologists for centuries, but there have been surprisingly few opportunities to test theoretical predictions in non-laboratory systems. We propose work on a tractable and well-studied wildlife-pathogen system, the house finch and its bacterial pathogen, Mycoplasma gallisepticum, in which we have already documented the independent evolution of increasing virulence since the emergence of the pathogen in two distinct parts of the host's range. We hypothesize two ecological conditions that underlie the observed patterns of increasing virulence evolution detected to date: 1) imperfect acquired host immunity, akin to vaccination, has selected for higher rates of within-host pathogen replication and associated higher virulence; and 2) higher contact rates, such as caused by anthropogenic feeding of birds, have resulted in increased virulence by minimizing the pathogen's risk of killing its host before it successfully transmits. The proposed work involves integration of theoretical virulence models with proposed experiments. First, formal within- and between-host models describing the two hypotheses will identify information needed for model refinement and validation; second, targeted experiments will quantify key parameters; and, third, these data will feed back to evaluate whether the models' assumed conditions explain observed field patterns. The empirical data will come from controlled infection and transmission experiments, which are unusually tractable in our study system, in order to characterize within- and between-host dynamics for partially immune versus immunologically naive hosts and at high and low between-host contact rates. Supplementing these model-focused experiments will be DNA.sequence and expression data from multiple field and experimental isolates of the bacteria, where our knowledge of gene function in M. gallisepticum will allow us to identify which genes may underiie the detected virulence changes. These data will serve as an independent test of the partial immunity hypothesis by addressing whether bacterial genes responsible for evading host immune systems are systematically changing in vivo and in vitro under selection due to acquired immunity, consistent with our hypothesis.