Objective: Elucidate the mechanisms underlying the loss of nasal T cells and characterize in depth the differences of T cells in young and older adults and associate this loss with susceptibility to infections. Study design: Prospective cohort study Study population: Participants will be recruited from 3 groups: * healthy young adults (18-30 years, n=50) * vital older adults (\>65 years, n=60) * frail elderly (\>65 years, n=60). This group includes individuals without a history of recurrent respiratory infections or with \>2 self-reported episodes of respiratory infection in the past year. Main study parameters/endpoints: Frequency of nasal CD8+ T cells in young adults and frail older adults. Secondary study parameters/endpoints: * Phenotype (subsets, activation status), functionality, transcriptomic state, clonality and frequency of nasal and blood T cell populations * Stability of T cells and other immune parameters, as described for main study parameter, during a second sample after 3 months. * Analysis of other immune populations as for main study parameter * Concentration of nasal and systemic factors (e.g. cytokines and metabolites) and their association with T cells and other immune populations * Respiratory tract microbiota profiles and presence of asymptomatic viral infections and their association with T cells and other immune parameters * Chronological and biological age, sex, and other immunologically relevant parameters with T cell populations and other immune parameters * Alteration of T cell phenotype, during and following respiratory tract infections. Levels of antigen-specific T cells and other immune parameters in nose and blood post infection.