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 two limiting cases for how host infectiousness may be determined: ‘Recipient dependence’ (RD), where a host's infectiousness is a fixed characteristic of the recipient host being infected, and ‘Donor dependence’ (DD), where a host's infectiousness is determined at the time of infection by properties of 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 the RD and DD scenarios generate distinctive epidemiological signatures. Although many host–parasite systems are likely to lie somewhere along a RD–DD continuum, with infectiousness influenced by both characteristics of the recipient and donor host, understanding these limiting cases provides a valuable framework for interpreting how host heterogeneity influences transmission in more complex systems.