Respiratory syncytial virus (RSV) is one of the most common causes of chest infection worldwide. Despite this, it remains an underappreciated health problem, with the first effective RSV vaccines only approved by the FDA in May 2023 and unlikely to be widely available for some time. Although RSV infection is most frequent in young children, most deaths occur in older adults, particularly in those with underlying heart and lung disease. This is believed to be due in part to the ageing immune system's reduced ability to protect against infection and symptomatic disease. However, little is known about the way human immune responses to RSV infection in older individuals differ from those of younger people. Further understanding of the mechanisms underlying immunity and potential impairments in these higher-risk people are therefore necessary. This project aims to study the factors that influence whether or not older people develop symptomatic RSV disease in healthy older volunteers after being given an RSV-induced common cold. Samples will be taken from the blood and nose in order to identify changes in the immune system associated with susceptibility or protection in older adults. Participants will be carefully screened to ensure there are no underlying health problems that might make them more at risk of severe disease and will be monitored closely throughout the course of infection. It is anticipated that differences in immune markers in the nose and/or blood of healthy older people will predict whether or not such individuals become infected or develop symptoms. By analysing the networks of genes that are switched on and off, the aim is to identify the pathways in the immune system responsible for these differences, to ultimately develop improved diagnostic tests, vaccines and treatments.