CD4 and infection: Since there was no change in absolute numbers of CD4+ T cells in the peripheral blood of infected volunteers during infection (S2 Fig), we hypothesized that the increase in the frequency and absolute numbers of CD38+ CD4+ T cells circulating during infection might result from the differentiation of naive CD4+ T cells into activated CD38+ CD4+ T cells, rather than from the proliferation of pre-existing CD38+ CD4+ T cells.