CD4 and infection: In this work, we investigate the causes and consequences of myeloid proliferation and activation during infection with P. chabaudi. We find that MCSF derived from multiple sources drives proliferation of macrophages and monocytes in infected mice; moreover, the full expansion and activation of certain subsets specifically requires MCSF derived from circulating CD4+ T cells, which have not previously been demonstrated to produce this cytokine in a physiological context.