| Sumario: | The thermal mismatch hypothesis (TMH), the notion that cool- and warm-adapted hosts have higher infection risk during unusually warm and cool spells, respectively, was recently proposed to explain how temperature shifts driven by climate change influence host susceptibility to infections at a global scale. Despite substantial support for the TMH in the chytrid fungus ( Batrachochytrium dendrobatidis , also commonly referred to as Bd) that is devastating amphibians worldwide, it remains unknown whether precipitation mismatches, in addition to temperature ones, affect infection risk. Here, we introduce the thermal-hydric mismatch hypothesis (THMH), which proposes that infection risk is shaped by mismatches resulting from the combined effects of temperature and precipitation. We tested this hypothesis using a large-scale survey of B. dendrobatidis and Ranavirus in over 5,800 adult amphibians across the Iberian Peninsula, a climate change hotspot in Europe that is heavily affected by both pathogens. We found that the combined effect of thermal and precipitation mismatches increased infection risk for both pathogens. Cool-and-wet-adapted amphibians were more infected with B. dendrobatidis during warm-and-dry spells, while warm-and-dry-adapted hosts showed higher B. dendrobatidis infection during cool-and-wet spells. For Ranavirus , mismatches occurred under opposite climatic conditions to those of B. dendrobatidis , which is consistent with the limited temporal overlap between the two pathogens, despite their frequent geographical co-occurrence. Finally, the fact that thermal mismatches alone did not predict B. dendrobatidis infection, and precipitation mismatches alone did not predict Ranavirus infection, suggests that infection risk in the study region is driven by their combined effects, supporting the THMH proposed here.
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