Department of Mathematics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.|Department of Mathematics, University at Buffalo, State University of New York, Buffalo, New York, United States of America.|Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.|Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, Michigan, United States of America.|Department of Mathematics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.|Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.|Department of Mathematics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.