<p>Upper respiratory tract infection (URTI), which presents clinically as the common cold, is the most common form of respiratory illness experienced by humans and is a major cause of morbidity and hospitalizations. Currently, URTI prevention focuses on hand hygiene with some consideration given to other lifestyle factors such as sleep, nutrition, and exercise. Identifying additional interventions for the prevention of URTI could reduce the burden of this disease.</p> <p>In this thesis, I examine the role of vitamin D3 supplementation and tap water gargling for the prevention of URTI. I employ experimental and observational study designs to assess the effect of these interventions on the risk of URTI in the context of a randomized controlled trial of 600 participants, and a longitudinal cohort of 416 participants. Further, I investigate the association between modifiable lifestyle factors and risk of URTI using data from the longitudinal cohort. Data from this study is also used to explore statistical methods for the analysis of repeated events.</p> <p>When evaluating self-reported, clinical URTI, all analyses supported the use of vitamin D3 supplementation to reduce the risk of URTI. However, this finding was only statistically significant in the analysis of the longitudinal cohort study; results from the RCT indicated that vitamin D3 supplementation statistically significantly reduced the risk of laboratory confirmed infections but had a non-significant benefit for clinical infections. Gargling did not reduce the risk of clinical or laboratory confirmed infections.</p> / Doctor of Philosophy (PhD)