Virus replication

In humans, the primary targets for influenza viruses are the epithelial cells in the airways and lungs. The viral HA binds to sialic acid residues on glycoproteins or glycolipids on the cell surface. The act of binding draws the virus into the cell membrane. The virus shell fuses with the cell membrane and moves through it, emerging into the cytoplasm of the cell, where the shell opens, releasing the RNA inside. The viral RNA is “negative sense” RNA – a mirror image of the messenger RNA the cell uses to make its own proteins. The RNA moves into the nucleus of the cell, where the cell’s machinery makes positive copies that travel back out into the cytoplasm. The cell treats them like any other messenger RNA and uses them to make copies of the viral proteins. At the same time, other positive copies of the viral RNA inside the nucleus act as templates to make more negative viral RNA. The new viral RNA then moves back into the cytoplasm where it joins with the newly made proteins to form new copies of the complete virus. The assembly occurs on the inside of the cell membrane, and as the process is completed, the virus moves through the cell wall and “buds” on the outside. The new virus is either released into the airway to find another cell to infect or is ejected in a cough or sneeze and launched to find a new host.