Colonization of the GI tract by SFB occurs in rat and mouse hosts at the onset of the weaning process, and thus SFB populations are low in neonates, which may contribute to their inability to mount IL-17 responses and their poor resistance to Giardia infection (Koopman et al., 1987; Ericsson et al., 2014). Here, IL17A is linked to giardiasis.