Parasite transmission is thought to depend on both parasite exposure and host susceptibility to infection; however, the relative contribution of these two factors to epidemics remains unclear. We used interactions between an aquatic host and its fungal parasite to evaluate how parasite exposure and host susceptibility interact to drive epidemics. In six lakes, we tracked the following factors from pre-epidemic to epidemic emergence: 1) parasite exposure (measured observationally as fungal spores attacking wild-caught hosts), 2) host susceptibility (measured experimentally as the number of fungal spores required to produce terminal infection), 3) host susceptibility traits (barrier resistance and internal clearance, both quantified with experimental assays), and 4) parasite prevalence (measured observationally from wild-caught hosts). Tracking these factors over six months and in almost 7,000 wild-caught hosts provided key information on the drivers of epidemics. We found that epidemics depended critically on the interaction of exposure and susceptibility; epidemics only emerged when a host population's level of exposure exceeded its individuals' capacity for recovery. Additionally, we found that host internal clearance traits (the hemocyte response) were critical in regulating epidemics. Our study provides an empirical demonstration of how parasite exposure and host susceptibility interact to inhibit or drive disease in natural systems and demonstrates that epidemics can be delayed by asynchronicity in the two processes. Finally, our results highlight how individual host traits can scale up to influence broad epidemiological patterns.

中文翻译：

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寄生虫暴露和宿主易感性共同推动流行病的出现

寄生虫传播被认为取决于寄生虫暴露和宿主对感染的易感性；然而，这两个因素对流行病的相对贡献仍不清楚。我们使用水生宿主与其真菌寄生虫之间的相互作用来评估寄生虫暴露和宿主易感性如何相互作用以驱动流行病。在六个湖泊中，我们跟踪了从流行前到流行病出现的以下因素：1）寄生虫暴露（观察性测量为真菌孢子攻击野生捕获的宿主），2）宿主易感性（实验测量为产生所需的真菌孢子数量）终感染），3）宿主易感性特征（屏障抗性和内部清除，均通过实验测定量化），和 4）寄生虫流行（从野生捕获的宿主中观察性测量）。在六个月内对近 7,000 个野生动物宿主中的这些因素进行跟踪，提供了有关流行病驱动因素的关键信息。我们发现流行病严重依赖于暴露和易感性的相互作用；只有当宿主人群的暴露水平超过其个体的恢复能力时，才会出现流行病。此外，我们发现宿主内部清除特性（血细胞反应）在调节流行病中至关重要。我们的研究提供了寄生虫暴露和宿主易感性如何相互作用以抑制或驱动自然系统疾病的实证证明，并证明流行病可以通过这两个过程的不同步性来延迟。最后，我们的结果强调了个体宿主特征如何扩大以影响广泛的流行病学模式。