Chronic colonisation of the lungs with Pseudomonas aeruginosa in patients with cystic fibrosis (CF) is associated with reduced lung function and life expectancy. Prevention of chronic colonisation might be achieved by avoidance of, or early and aggressive treatment of, primary P aeruginosa acquisition.¹ Segregation of uninfected indivduals from chronically P aeruginosa colonised CF patients is advocated to prevent cross infection.² As surveillance studies suggest that the airways of healthy children are rarely colonised with P aeruginosa,³ healthy individuals are not regarded as a potential source of P aeruginosa acquisition. In addition, it has been shown that acquisition of P aeruginosa in CF patients is often preceded by a viral respiratory infection.⁴ We hypothesised that the incidence of P aeruginosa acquisition during periods of acute respiratory infections (ARI) is equal in both healthy and CF individuals, and considerably exceeds the prevalence in …