We use the epidemic threshold parameter, R0\documentclass[12pt]{minimal}
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\begin{document}$${{\mathcal {R}}}_{0}$$\end{document}, and invariant rectangles to investigate the global asymptotic behavior of solutions of the density-dependent discrete-time SI epidemic model where the variables Sn\documentclass[12pt]{minimal}
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\begin{document}$$S_{n}$$\end{document} and In\documentclass[12pt]{minimal}
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\begin{document}$$I_{n}$$\end{document} represent the populations of susceptibles and infectives at time n=0,1,…\documentclass[12pt]{minimal}
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\begin{document}$$n = 0,1,\ldots $$\end{document}, respectively. The model features constant survival “probabilities” of susceptible and infective individuals and the constant recruitment per the unit time interval [n,n+1]\documentclass[12pt]{minimal}
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\begin{document}$$[n, n+1]$$\end{document} into the susceptible class. We compute the basic reproductive number, R0\documentclass[12pt]{minimal}
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\begin{document}$${{\mathcal {R}}}_{0}$$\end{document}, and use it to prove that independent of positive initial population sizes, R0<1\documentclass[12pt]{minimal}
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\begin{document}$${{\mathcal {R}}}_{0}<1$$\end{document} implies the unique disease-free equilibrium is globally stable and the infective population goes extinct. However, the unique endemic equilibrium is globally stable and the infective population persists whenever R0>1\documentclass[12pt]{minimal}
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\begin{document}$${{\mathcal {R}}}_{0}>1$$\end{document} and the constant survival probability of susceptible is either less than or equal than 1/3 or the constant recruitment is large enough.
