The unprecedented loss of biological diversity from anthropogenic causes has profound impacts on human health. One way that biodiversity loss threatens human health is by exacerbating risk and incidence of infectious diseases. This paper briefly reviews two zoonotic diseases--West Nile virus (WNV) illness and Lyme disease (LD)--in which high diversity in the community of vertebrate hosts for arthropod vectors strongly reduces human risk. In both cases, the primary reservoirs for the pathogen are species that dominate in human-impacted, low-diversity communities. As a result, the generalist vectors responsible for transmitting the pathogens to humans have relatively high feeding rates on these reservoirs, leading to high infection prevalence in mosquito (for WNV) and tick (for LD) vectors. In contrast, where native vertebrate diversity is high, mosquito and tick vectors evidently feed from a wider variety of hosts, most of which are poor reservoirs for the pathogens, resulting in lower infection prevalence. Protection of humans against exposure to zoonotic pathogens should be added to the list of utilitarian functions provided by high biodiversity.