1. Department of Infectious Diseases and Microbiology, Imperial College School of Medicine, St Mary's Campus, Norfolk Place, London W2 1PG, UK
2. Department of Biosciences, University of Hertfordshire, Hatfield, Herts AL10 9AB, UK
3. Faculty of Continuing Education, Birkbeck College, 26 Russell Square, London WC1B 5DQ, UK
4. Department of Respiratory Medicine, Imperial College School of Medicine, St Mary's Campus, Norfolk Place, London W2 1PG, UK