The ratio of protective proinflammatory cytokines (such as IFN-γ and TNF-α) to regulatory cytokines (such as IL-10 and TGF-β), has been shown to determine the outcome of natural blood-stage infection with lethal versus nonlethal P. yoelii (Omer et al., 2003) and to correlate with the growth rate of blood-stage parasites in malaria-naive human volunteers (Walther et al., 2006). The gene discussed is IL10; the disease is malaria.