“…This function, which has its origin in ecology, was extended to the models of epidemiology by Korobeinikov and Wake (2002) and Korobeinikov (2004a), and then effectively applied to a variety of compartment models, including the models with multiple progressive stages (Guo and Li, 2006;Korobeinikov, 2004b), and models with nonlinear incidence rates of different forms (Georgescu and Hsieh, 2006;Korobeinikov, 2006Korobeinikov, , 2007Korobeinikov, , 2008Maini, 2004, 2005;Korobeinikov and Petrovskii, 2008).…”