Pii: S0025-5564(00)00006-7

A.S. Perelson, D.E. Kirschner and R. De Boer (Math. Biosci. 114 (1993) 81) proposed an ODE model of cell-free viral spread of human immunode®ciency virus (HIV) in a well-mixed compartment such as the bloodstream. Their model consists of four components: uninfected healthy CD4‡ T-cells, latently infected CD4‡ T-cells, actively infected CD4‡ T-cells, and free virus. This model has been important in the ®eld of mathematical modeling of HIV infection and many other models have been proposed which take the model of Perelson, Kirschner and De Boer as their inspiration, so to speak (see a recent survey paper by A.S. Perelson and P.W. Nelson (SIAM Rev. 41 (1999) 3±44)). We ®rst simplify their model into one consisting of only three components: the healthy CD4‡ T-cells, infected CD4‡ T-cells, and free virus and discuss the existence and stability of the infected steady state. Then, we introduce a discrete time delay to the model to describe the time between infection of a CD4‡ T-cell and the emission of viral particles on a cellular level (see A.V.M. Herz, S. Bonhoe€er, R.M. Anderson, R.M. May, M.A. Nowak [Proc. Nat. Acad. Sci. USA 93 (1996) 7247]). We study the e€ect of the time delay on the stability of the endemically infected equilibrium, criteria are given to ensure that the infected equilibrium is asymptotically stable for all delay. Numerical simulations are presented to illustrate the results. Ó 2000 Elsevier Science Inc. All rights reserved.