Warmer world gets sicker: meta-analysis reveals strong increase in parasitism at elevated temperatures across diverse host-parasite systems

Climate change is expected to alter infectious disease dynamics worldwide, yet evidence that rising temperatures increase infection (warmer, sicker world hypothesis; WSWH) remains equivocal. We conducted a meta-analysis of 775 effect sizes from 124 experimental studies spanning terrestrial, freshwater, and marine host-parasite systems to test whether warming increases parasitism. Elevated temperatures increased infection, producing moderate positive effects on prevalence and infection intensity in non-phylogenetic models. Temperature effects were significant in terrestrial systems and varied markedly among taxa. Strong responses were observed in nematodes and fungal parasites as well as in plant, invertebrate, and bacterial hosts, whereas vertebrates showed little evidence of consistent increases. Phylogenetically-controlled analyses yielded similar but less precise estimates, indicating that responses are unevenly distributed across the Tree of Life. Our synthesis provides the most comprehensive experimental test of the WSWH to date and shows that warming can elevate infection risk while generating heterogeneous outcomes among host-parasite systems.