Coevolution of Hosts and Pathogens in the Presence of Multiple Types of Hosts

How will hosts and pathogens coevolve in response to multiple types of hosts? I study this question from three different perspectives. First, I model a scenario in which hosts are categorized as female or male. Hosts invest resources in maintaining their immune system at a cost to their reproductive success, while pathogens face a trade-off between transmission and duration of infection. Importantly, female hosts are also able to vertically transmit an infection to their newborn offspring. The main result is that as the rate of vertical transmission increases, female hosts will have a greater incentive to pay the cost to invest in their immune system, while the pathogen will evolve a lower rate of disease-induced mortality in female hosts relative to male hosts. Second, I study a model where hosts can change their type. Hosts are classified according to whether they engage or do not engage in prophylactic behaviours that reduce the transmission rate of an infectious disease and may freely start or stop these behaviours. I study the evolution of the degree to which the pathogen exploits its host’s resources. The main result is that, when hosts engage in prophylaxis, I expect pathogens to evolve a lower level of host exploitation than would otherwise be predicted in the absence of prophylactic behaviours. Finally, I consider the possibility of multiple pathogen strains in addition to multiple types of hosts. I develop a general theoretical framework to link ideas from evolutionary epidemiology to those from kin selection theory to study the evolution of coinfecting pathogen strains. The main result is that pathogens will evolve to balance both the direct and indirect benefits of increased transmission with the associated direct and indirect costs of decreased duration of infection within each type of host. The more genetically related is a pathogen strain to its coinfecting group, the more it will reduce its own disease-induced mortality rate for the benefit of the whole group. Overall, this work shows how standard results from evolutionary epidemiology change when considering the coevolution of hosts and pathogens in the presence of multiple types of hosts