The study of personality - consistent individual differences in animal behaviour - is a rapidly expanding research field. Like humans, animals may be categorized on personality axes such as bold-shy and aggressive-passive. These traits have both costs and benefits. However, the mechanisms underlying individual variation in personality and the consequences for evolutionary fitness are largely unknown.For the first time, links between personality and stress physiology (a potential determinant of personality) will be investigated in the context of parasite infection. Using a fish host-gyrodactylid parasite model, this fellowship will (1) quantify the links between personality and stress, (2) assess the susceptibility to infection of hosts that vary in personality/stress physiology, and (3) determine how environmental stress during development affects adult personality and subsequent parasite infection.This multi-disciplinary project will integrate techniques from behavioural ecology, physiology and parasitology, to provide a 'step change' in our understanding of the causes and consequences of personality. Gyrodactylids are important ectoparasites, ubiquitous on teleost fish, which have huge economic impacts, particularly in the Norwegian salmon industry. Knowledge of how stress during development ultimately affects disease susceptibility, has important implications for fisheries, agriculture and captive-rearing programmes for conservation. However, the results from this project are applicable to disease dynamics in a wide range of hosts, including humans. Overall, determining how variation in host personality affects infection rates will provide a fundamental advance in our understanding of host-parasite dynamics.