Elucidating the determinants of parasite infection levels of individuals within a population is a basic but important step for understanding the roles of parasites in natural systems. Among intraspecific studies on vertebrate hosts and metazoan parasites, host body size (length/mass) is probably the most frequently reported factor determining parasite infection levels (intensity/abundance), where positive relationships are usually observed. However, whether the positive correlations between host size and infections represent a general rule remains unclear, as do the factors determining the strength of these correlations. Here, by conducting meta-analyses using 793 infection-versus-body size correlations compiled from 153 publications, we tested whether the positive relationship between host body size and parasite infections represents a general pattern across all vertebrate host taxa and major metazoan parasite groups. Results showed that overall correlations were significantly positive but relatively weak (Zr = 0.206), indicating that the presumed positive relationships between host body size and infections are not strongly supported as a general rule, and that the strength of correlations varied greatly among host-parasite systems. We also found that studies on mammals and birds were more likely to report non-significant correlations, possibly due to behavioral, developmental, and physiological characteristics, compared with those on fish. Moreover, effect sizes varied depending on study methods, such as infection or host body size measures used, suggesting that not only biological processes but also statistical noise associated with these methods influence the relationships. Our study provides the first quantitative evidence showing that positive relationships between host body size and infection are not a universal pattern and are relatively heterogeneous among host-parasite systems. Based on these findings, we stress the importance of testing rather than assuming host size-infection relationships in any given host-parasite system, and considering both biological and other underlying processes, to better understand the determinants of infection in nature.

Although parasites reduce host health, parasite infections also occur as a consequence of compromised host health. Both causalities could induce positive feedback, in which infected hosts with poor body conditions may suffer further infection, but it has rarely been demonstrated in the wild, possibly due to methodological difficulties. We used a mark-recapture survey combined with structural equation modelling (SEM) to examine whether both causalities and positive feedback occurred in stream salmonid and parasitic copepod systems. We found that parasitic copepods reduced host conditions and hosts with poor conditions were likely to be infected, suggesting that positive feedback can occur in the wild. Importantly, heavily infected hosts with poor body conditions showed lower apparent survival rates. Our findings provide robust evidence showing host condition–parasite infection dynamics, offering novel insights into how positive feedback could strongly undermine the wild host population via reduction of host survival.