Levy et al. employed equal populations of homozygous CFP and YFP reporter viruses to infect ∼106 CD4+ T cells and detected the total percentage of cells infected (i.e., that fluoresced), p*, the percentage of cells that were coinfected with CFP and YFP genomes, p12, and the percentage of cells that were GFP+, p4, with time following the onset of infection. The gene discussed is CD4; the disease is infection.