Humans may be susceptible or infected (H S and H I , resp.), while mosquitoes may be susceptible or infected (M S and M I , resp.). The birth rates are Λ H for humans and Λ M for mosquitoes. The background death rates are μ H and μ M for humans and mosquitoes, respectively, while the diseasespecific death rate for humans is σ. The transmission rate from mosquitoes to humans is β MH , while the transmission rate from humans to mosquitoes is β HM . See Figure 3 .