Figure 4. The Predicted Time for Memory CD4⁺ T Cell Numbers to Decline from 100% to 50% of Healthy Numbers, as a Function of Virus Infectivity p .

Parameter values (see model equations in caption to Figure 2): a(x) = a ₀(1 − x/κ), where a ₀ = 1/30 d⁻¹, giving a(x*) ≈ 1/60 (here x* refers to the steady state numbers of resting memory CD4⁺ T cells in an uninfected individual); κ = 1; δ(x) = δ ₀ x/κ, where δ ₀ = 1/200, giving δ(x*) ≈ 1/400; μ = 1.06; v = 0.5; and r = 1 (all rates in units of d⁻¹).