Parasite evolution and extinctions

We examine the evolution of diseases that show the frequency-dependent transmission process that is commonly applied to sexually and vector-transmitted infections. As is commonly found, the basic reproductive ratio (R-0) of the parasite is maximized by evolution. This has important implications, as it implies that for a wide range of circumstances diseases that show frequency-dependent transmission may be selected to evolve towards driving their hosts to extinction. This contrasts with the results obtained in spatially explicit models where although parasite-driven host extinction may occur, it is unlikely to evolve. We further show that an evolutionary constraint between transmission and virulence is required for evolution to lead to an endemic coexistence of both the host and the disease. Furthermore, this constraint needs to be saturating, such that transmission is 'bought' at an increasing cost in terms of virulence, to avoid evolution to extinction.