Parasite epidemics can depend on priority effects, and parasite priority effects can result from the host immune response to prior infection. Yet we lack experimental evidence that such immune-mediated priority effects influence epidemics. To address this research gap, we manipulated key host immune hormones, then measured the consequences for within-host parasite interactions, and ultimately parasite epidemics in the field. Specifically, we applied plant immune-signaling hormones to sentinel plants, embedded into a wild host population, and tracked foliar infections caused by two common fungal parasites. Within host individuals, priority effects were altered by the immune-signaling hormone, salicylic acid. Scaling up from within-host interactions, hosts treated with salicylic acid experienced lower prevalence of a less aggressive parasite, increased burden of infection by a more aggressive parasite, and experienced fewer coinfections. Together, these results indicate that by altering within-host priority effects, host immune hormones can drive parasite epidemics. This study therefore experimentally links host immune hormones to within-host priority effects and parasite epidemics, advancing a more mechanistic understanding of how interactions among parasites alter their epidemics.