CD4 and infection: An erythrocytic-stage P. chabaudi infection, the only mouse model that generates a chronic phase of infection (3), presents a biphasic CD4+ T-cell activation, with a large IFN-γ-producing CD4+ T-cell response during the acute phase, followed by an antibody-helper/IL-4-producing CD4+ T-cell response during the chronic phase (Figure 1C) (21, 24, 45, 46).