Illustration of a simplified host-parasite system that highlights temperature-109 dependent processes occur and interact across levels of biological organization. Temperature 110 can influence parasite growth and host defenses against infection, which can ultimately lead to a 111 thermal response for individual-level parasitism (e.g., parasite burden; a). Similarly, temperature 112 can affect contact rates between hosts or hosts and parasites, the probability of infection after 113 contact, and host density, all of which can lead to variation in parasite transmission rate across 114 temperature (b). In some cases, the thermal response of individual-level parasitism (a) may also 115 partially influence the thermal response of parasite transmission (b), as higher parasite burden 116 can confer a higher probability of infection after contact or can alter contact rates between hosts. 117 Together, the thermal responses of individual-level parasitism (a) and parasite transmission (b) 118 will help determine the thermal response of population-level parasitism (e.g., parasite prevalence 119 or the basic reproduction number R0; c). 120 121 122 123 124 125 126 127 128 129 130 131 132

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... These and other outbreaks have made predicting effects of temperature and changing 33 climate on infectious disease dynamics an urgent priority (Altizer et al. 2013). Importantly, 34 temperature affects biological systems across levels of organization, from individual physiology 35 and infection burden to host population and disease dynamics ( Fig. 1). This means that 36 understanding the effects of temperature on both individuals and populations is critical for 37 disease mitigation efforts as the world warms. Yet this cross-scale investigation of thermal 38 disease ecology remains an open research gap. ...

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... processes and the traits that 43 underlie them may be temperature dependent. One challenge for anticipating severe 44 consequences of parasitism is to understand how temperature effects emerge at the host 45 population level even when there can be a range of thermal responses among other biological 46 processes influencing the host and parasite (Fig. 1). For example, multiple host and parasite traits 47 and their respective temperature dependencies (typically described using thermal performance 48 curves, or TPCs) influence the effects of temperature on individual-level parasitism (Kirk et al. 49 2018). Moreover, individual-level parasitism outcomes, such as parasite burden (McCallum ...

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... parasitism is unlikely to be 56 so straightforward. Several potentially temperature-dependent traits can affect parasite 57 transmission, including demographic traits that influence host population dynamics and 58 behavioral traits of hosts, and these traits can therefore alter how individual-level parasitism 59 scales to the population level ( Fig. 1). For example, activity rate of individuals can vary with 60 temperature (Casey 1976), thereby likely modifying the contact rate between susceptible and 61 infected individuals and the overall transmission rate. Additionally, the density of susceptible 62 hosts, often a key factor in parasite transmission, depends on demographic ...

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... framework for understanding temperature-driven changes in disease centers on the 71 thermal mismatch hypothesis . Complementary to the approach we outline 72 above and in Figure 1, which emphasizes parasitism at the host individual and host population 73 levels, the thermal mismatch hypothesis relates the thermal response of parasitism to that of the 74 host. The thermal mismatch hypothesis predicts that parasitism is maximized at temperatures 75 away from the host's optimum-i.e., at cool temperatures for warm-adapted species and at warm 76 temperatures for cold-adapted species-at both the host individual and host population levels 77 (i.e., individual-and population-level parasitism TPCs should peak at temperatures offset from 78 the host optimum in what is called a thermal mismatch; Cohen et al. 2017Cohen et al. , 2019a. ...

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... impacts parasitism at the individual host level by influencing parasite growth and host defenses against infection, ultimately leading to differential levels of parasite burden across temperature. This thermal response of individual-level parasitism is intrinsically tied to the thermal response of parasitism at the population level (Fig. 1, 4). In our examination of thirteen empirical systems, we found a significant positive relationship between Topt for parasitism at the two levels (Fig. 2). Additionally, while individual-and population-level parasitism both peaked at temperatures away from the host optimum in some systems, supporting the thermal mismatch hypothesis, this ...

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... that in the majority of systems, population-level parasitism tended to peak at warmer temperatures than individual-level parasitism (Fig. 2). The greatest differences in Topt were observed in the zooplankton D. magna -O. colligata system, cold-adapted amphibians -B. dendrobatidis system, and the zooplankton D. dentifera -M. bicuspidata system (Fig. 2, Table 1). It is difficult to definitively parse out the causes of Topt differences in the two amphibian systems since individual-level Bd parasitism was measured on either one or two frog species in the lab while population-level prevalence by the same parasite was synthesized across many field studies encompassing 235 host species ...