1. School of Mathematics and Statistics, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China
2. Department of Integrative Biology, The University of Texas at Austin, Austin, TX 78712, USA
3. Department of Mathematics and Statistics, Oakland University, Rochester, MI 48309, USA
4. Department of Mathematics, University of Florida, Gainesville, FL 32611, USA
5. The Santa Fe Institute, Santa Fe, NM 87501, USA
6. Department of Epidemiology and Biostatistics, College of Public Health, University of Georgia, Athens, GA 30602, USA
7. Center for Ecology of Infectious Diseases, University of Georgia, Athens, GA 30602, USA