<P> Viruses can replicate only in living cells . Influenza infection and replication is a multi-step process: First, the virus has to bind to and enter the cell, then deliver its genome to a site where it can produce new copies of viral proteins and RNA, assemble these components into new viral particles, and, last, exit the host cell . </P> <P> Influenza viruses bind through hemagglutinin onto sialic acid sugars on the surfaces of epithelial cells, typically in the nose, throat, and lungs of mammals, and intestines of birds (Stage 1 in infection figure). After the hemagglutinin is cleaved by a protease, the cell imports the virus by endocytosis . </P> <P> The intracellular details are still being elucidated . It is known that virions converge to the microtubule organizing center, interact with acidic endosomes and finally enter the target endosomes for genome release . </P> <P> Once inside the cell, the acidic conditions in the endosome cause two events to happen: First, part of the hemagglutinin protein fuses the viral envelope with the vacuole's membrane, then the M2 ion channel allows protons to move through the viral envelope and acidify the core of the virus, which causes the core to disassemble and release the viral RNA and core proteins . The viral RNA (vRNA) molecules, accessory proteins and RNA - dependent RNA polymerase are then released into the cytoplasm (Stage 2). The M2 ion channel is blocked by amantadine drugs, preventing infection . </P>

Where is the flu found in the world