Figure 1.

Potential roles that commensal microbes within the intestine may play in host susceptibility to virus. (A) A commensal may inhibit a virus (top left panel) directly (a), or indirectly at a local site (b), or at a peripheral site such as the lung (c). Conversely, this same commensal may promote viral infectivity (lower left panel) via these same mechanisms, such as directly activating the virus (a), recruiting leukocytes that promote/permit viral replication (b), or setting systemic immune parameters that are permissive for viral replication at a distant site (c). (B) Other commensals may have opposite effects on the same viruses, and alterations in microbiota composition or functional state may shift the balance in favor of the first or second scenario.