1. Section of Comparative Medicine and Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven, Connecticut 06510, USA
2. Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven, Connecticut 06510, USA
3. Division of Wildlife and Ecology, Commonwealth Scientific and Industrial Research Organization, Canberra, Australian Capital Territory 2602, Australia
4. Department of Microbiology, The University of Western Australia, Nedlands, Perth 6009, Australia