 Struggle and cooperation in the immune context were assumed since Koch to depend on intrinsic characteristics of invading microbes that, due to their virulence factors and other gene-encoded characteristics, act as pathogens, commensals or mutualists (Méthot and Alizon 2014). By accounting for a larger context of the immune encounter, the ecological position challenges this vision by offering a view in which “there are no pathogens, no commensals, and no opportunists; instead there are only microbes and hosts that interact” unpredictably (Casadevall and Pirofski 2015). In this scenario, microbial virulence becomes an emergent property, dependent on host and microbial factors, whose interactions cannot be derived from first principles (Casadevall, Fang, and Pirofski 2011).