When a host population is faced with a pathogen it may invest resources in avoiding infection, recovering from infection or developing immune memory against the infection. All of these mechanisms of costly host defence decrease the incidence of infection (in contrast to tolerance) and are therefore known as host resistance. Because investment in resistance lowers the incidence of infection which in turn decreases the advantage of resistance, an evolving population will often come to rest at an intermediate level of investment. Optimal levels of investment in costly resistance reflect a fine balance between epidemiological and evolutionary dynamics.

To begin with we demonstrate key qualitative differences in the nature of this balance depending on the combination of host and pathogen types. We then return to some important questions in the study of resistance. Is high investment associated with high incidence of infection or high force of infection? How does optimal investment vary between populations? How are these patterns affected by concurrent pathogens residing in the host population? We show how exact invasion fitness and approximations for optimal investment can be used to answer these questions.

density dependence; resistance; epidemiology; optimal evolutionary investment; evolutionary invasion analysis