Simon L Croft1, Steven Ward1. 1. Simon L. Croft is Professor of Parasitology at the London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK. E-mail: simon.croft@lshtm.ac.ukSteven Ward is the Walter Myers Professor of Parasitology at the Liverpool School of Tropical Medicine, Pembroke Place, Liverpool L3 5QA, UK. E-mail: steve.ward@lstmed.ac.uk.