<i>Giardia</i> infection expanded IL-10-producing Th2 and GATA3<sup>+</sup> Treg cells that promoted chronic carriage, parasite transmission, and conferred protection against <i>Toxoplasma gondii</i>-induced lethal ileitis and DSS-driven colitis by downregulating proinflammatory cytokines, decreasing Th1/Th17 cell frequency, and preventing collateral tissue damage. Here, IL10 is linked to infection.