The consequences of host heterogeneity for parasite transmission depend on how host infectiousness is determined

Host heterogeneities in susceptibility and infectiousness can affect parasite transmission, but standard analyses typically consider a limited range of epidemiological scenarios. We propose two contrasting phenomenological scenarios that capture a wide range of processes by which host infectiousness is determined, and explore how these scenarios affect the impact of host heterogeneities on parasite transmission. Specifically, we contrast ‘Recipient dependence’ (RD), where a host’s infectiousness is a fixed characteristic of the recipient host being infected, with ‘Donor dependence’ (DD), where a host’s infectiousness is determined by the donor host that infected them. Contrasting Susceptible-Infected models of these two phenomenological scenarios show that under RD, R ₀ is driven by population-level covariance between susceptibility and infectiousness, but for DD scenarios it is driven by the maximum infectiousness present in the population. Our results show that these different scenarios, which capture different ways by which host infectiousness is determined, should be considered explicitly when exploring the consequences of host heterogeneity on transmission.