Figure 1.

Influenza A virus, an enveloped, negative-sense RNA virus, enters cells by endocytosis. Endocytic maturation exposes the virus to stages of acidification. Fusion of viral and endosomal membranes occurs most rapidly at pH 5; however, studies have shown that the HA fusion protein becomes active at pH 5.5–6 to begin the process of membrane remodeling that ultimately leads to fusion.