1. Department of Zoology, Oxford University, Oxford OX1 3PS, UK
2. Department of Disease Control, London School of Hygiene & Tropical Medicine, London WC1E 7HT, UK
3. Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, University of North Carolina at Chapel Hill, NC 27599, USA
4. Water Institute, Gillings School of Global Public Health, University of North Carolina at Chapel Hill, NC 27606, USA