The broad aims of this thesis were to investigate the influence of vitamin D supplementation on URTI burden in military recruits(Chapter 3), to examine the influence of psychosocial and behavioural factorson URTI riskin marathon runners(Chapter 4), and to investigate the influence of psychological stress and anxiety on the mucosal immune response to exercise, in both acontrolled, lab-based setting and afield-basedmarathon (Chapter 5).Firstly, we found that vitamin D sufficiency lowered URTI burden in military recruits during Army training (Chapter 3). Specifically, in 1,644 military recruits, we found that vitamin D sufficient recruits were less likely to have a clinician-diagnosed URTI during training, compared to those with serum 25(OH)D <50 nmol·L-1(Study 1). Then, in a randomised-control-trial (RCT),including 249 men, we found that vitamin D supplementation by oral D3or simulated sunlight, which achieved vitamin D sufficiency in almost all, reduced URTI burden regardless of supplementation type: 21% lower URTI peak severity and 36% fewer days with URTI(Study 2). And thatindividuals beginning supplementation with 25(OH)D <50 nmol·L-1benefittedfrom greater reductions in URTI burden: 33% shorter URTI duration. However, we did not find any influence of vitamin D supplementation on mucosal immunity. In Chapter 4, a prospective, cohort study of 305 marathon runners, weexamined the influence of psychosocial factors and sleepon URTI prevalence. We found that runners were more likely to report a URTI during the two weeks before or after a marathon if they reportedhigher psychological stress, anxiety or neuroticism, early life adverse experience, or lower perceived sleep quality. Runners with early life adverse experience were over two times more likely to report a URTI during the two weeks beforethe marathon(OR: 2.33)and runners vwith poorer perceived sleep quality were two times more likely to report a URTI during the two weeksafterthe marathon(OR: 0.48). In the final empirical chapter, we examined the influence of psychological stress and anxiety on the mucosal immune response to a moderate, lab-basedbout of exercise (Study 1) and a field-basedmarathon (Study 2; Chapter 5). We found that perceived psychological stress, trait anxiety and state anxiety influencedthe mucosal immune response to exercise. In men, perceived psychological stress, trait anxiety and state anxiety were negatively correlated with the mucosal immune response toexercise. In response to a controlled, moderate bout of exercise, in individuals with predominantly low-moderate psychological stress and anxiety, men with lower psychological stress and anxiety experienced an increase in saliva SIgA SR, whereas those with moderate stress and anxiety saw little change. Then, in a field-based marathon, when includingindividuals reportinghigh stress and anxiety, men with higher perceived psychological stress and trait anxiety had a greater reduction in mucosal immunityin response to a marathon. These findings support the recommendation that exercise physiologists should account for psychological stress and anxiety when examining the immune response to exercise. Taken together, Chapters4 and5indicate a key role of psychological factors in immunity in exercising individuals.Specifically, individuals with higher psychological stress and anxiety appear to be at greater risk of URTI and experience greater reduction in mucosal immunity following exercise.