1 Christ Church, Department of Zoology, and Mathematical Institute, University of Oxford, Oxford, UK.
2 Department of Food Hygiene and Environmental Health, and Department of Mathematics and Statistics, University of Helsinki, Helsinki, Finland.
3 School of Public Health and Porter School of the Environment and Earth Sciences, Tel Aviv University, 69978 Tel Aviv, Israel. Electronic address: uriobols@tauex.tau.ac.il.