Respiratory viruses including influenza and respiratory syncytial virus (RSV) are among the most important causes of severe disease globally, infecting everyone repeatedly throughout life. Understanding of how to prevent infection is incomplete but boosting immunity with vaccines remains the best strategy. T cells have been shown in animals to be essential for clearing respiratory viral infection and are likely to be helpful if stimulated by vaccines. However, where these cells originate from and how they develop in the human lung are still unclear. The investigators will inoculate volunteers with influenza or RSV to examine the relationship between T cells in their blood and lungs and the outcome of infection. By tracking these specialised cells, the investigators aim to develop a better understanding of how they are generated in order to harness them with future vaccines.