Fig. 4B depicts the number of CD4+ T cells newly infected via either cell-to-cell spread or cell-free spread as the infection progresses. The model predicts that cell-to-cell transfer becomes increasingly dominant as the total number of CD4+ T cells falls, the proportion of susceptible cells rises (Fig. 4B inset left y axis) and the strength of immune response falls (because of immune exhaustion, see Fig. 4B inset right y axis). The gene discussed is CD4; the disease is infection.