Published online by Cambridge University Press: 14 December 2020 Arni S.R. Srinivasa Rao Affiliation: Professor, Medical College of Georgia Director, Laboratory for Theory and Mathematical Modeling, Division of Infectious Diseases, Department of Medicine, Medical College of Georgia, Augusta, GA 30912, U.S.A. and Professor, Department of Mathematics, Augusta University, Augusta, U.S.A. Steven G.