Parasitology 2004-0226, revised on 22 December 2004 PAGE 1 Density dependence and overdispersion in the transmission of helminth parasites T. S. CHURCHER*, N. M. FERGUSON and M-G. BASÁÑEZ Department of Infectious Disease Epidemiology, Faculty of Medicine, St Mary’s campus, Imperial College London, Norfolk Place, London W2 1PG, UK * Corresponding author: Department of Infectious Disease Epidemiology, Imperial College London, Norfolk Place, London W2 1PG, UK. Tel: + 44 (0) 20 75943229; Fax: + 44 (0)20 74023927; E-mail: thomas.churcher@imperial.ac.uk