In support of this work, our laboratory previously showed that TLR4−/− mice are protected against infection by mouse-adapted A/PR/8/34 (PR8) influenza virus and that therapeutic administration of a potent, synthetic TLR4 antagonist, Eritoran (also known as E5564), blocks PR8-induced lethality, lung pathology, clinical symptoms, and cytokine production in wild-type mice (12, 13). Here, TLR4 is linked to infection.