1. Department of Demography, University of California, Berkeley , Berkeley, California , USA
2. Division of Epidemiology, University of California, Berkeley , Berkeley, California , USA
3. Department of Epidemiology, School of Public Health, Brown University , Providence, Rhode Island , USA
4. Institute for Environment and Society, Brown University , Providence, Rhode Island , USA
5. Department of Microbiology, University of Illinois Urbana-Champaign , Champaign, Illinois , USA
6. Department of Statistics, University of Illinois Urbana-Champaign , Champaign, Illinois , USA