Data and code from: Mixed infection, risk projection and misdirection: Interactions among pathogens alter links between host resources and disease

A growing body of literature links resources of hosts to their risk of
infectious disease. Yet most hosts encounter multiple pathogens, and
projections of disease risk based on resource availability could be
fundamentally wrong if they do not account for interactions among
pathogens within hosts. Here, we measured infection risk of grass hosts
(Avena sativa) exposed to three naturally-co-occurring viruses either
singly or jointly (barley and cereal yellow dwarf viruses [B/CYDVs]:
CYDV-RPV, BYDV-PAV, and BYDV-SGV) along experimental gradients of nitrogen
and phosphorus supply. We asked whether disease risk (i.e., infection
prevalence) differed in single versus co-inoculations, and whether these
differences varied with rates and ratios of nitrogen and phosphorus
supply. In single inoculations, the viruses did not respond strongly to
nitrogen or phosphorus. However, in co-inoculations, we detected
illustrative cases of 1) resource-dependent antagonism (RPV with
increasing N; possibly due to competition), 2) resource-dependent
facilitation (SGV with decreasing N:P; possibly due to immunosuppression),
and 3) weak or no interactions within hosts (for PAV). Together, these
within-host interactions created emergent patterns for co-inoculated
hosts, with both infection prevalence and viral richness increasing with
the combination of low nitrogen and high phosphorus supply. We demonstrate
that knowledge of multiple pathogens is essential for predicting disease
risk from host resources, and that projections of risk that fail to
acknowledge resource-dependent interactions within hosts could be
qualitatively wrong. Expansions of theory from community ecology theory
may help anticipate such relationships linking host resources to diverse
pathogen communities.