This mixed methods study informed the development of our web-based intervention to reduce the risk of transmission of respiratory infections. Two qualitative studies examined 28 users’ responses to website materials, and elicited additional beliefs. A questionnaire study ( N = 129) tested key intervention assumptions: that hand-washing is considered the most feasible preventive behaviour; behavioural determinants were related to behavioural intentions; beliefs related to hand-washing intentions and behaviour. Thematic analysis of the qualitative data revealed important issues, such as the belief that catching minor infections is beneficial to the immune system. The questionnaire results confirmed our assumptions, including hand-washing as the appropriate target behaviour.