1. Center for Infectious Disease Dynamics, Department of Biology, The Pennsylvania State University, Mueller Laboratory, University Park, PA 16802, USA
2. J.A. Baker Institute, Department of Microbiology and Immunology, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853, USA
3. Fogarty International Center, National Institutes of Health, Bethesda, MD 20892, USA