We first stained peripheral blood mononuclear cells (PBMCs) with a proliferation tracking dye, CellTrace Violet (CTV), and then infected with EBV at a multiplicity of infection such that every CD19+ B cell is latently infected with EBV.21 At 4 days post infection, we sorted the majority of infected PBMCs for CD19 positivity and dilution of CTV thereby isolating a purified population of infected, early rapidly proliferating B cells (Figure 1a). Here, CD19 is linked to infection.