CD4 and HIV infectious disease: To be able to incorporate each of these processes responsible for CD4+ T cell maintenance for an uninfected individual, and later when HIV infection is included, we divided memory CD4+ T cells in the model into 4 subsets (Fig 2 and S1 Text): resting and non-dividing (R), resting and dividing (R+, encompassing cells undergoing homeostatic proliferation), activated and non-dividing (A), and activated and dividing (A+, encompassing cells undergoing antigenic proliferation).