Cell Invasion
As Russell Kightley Media explains, the first step in the influenza virus life cycle is cell invasion, which typically initially occurs in the cells that line the lungs. This cell invasion involves the docking of special proteins called hemagglutinins (found on the surface of the virus) to the respiratory cell membrane. Once this occurs, the virus is imported into the cell via a process called endocytosis, which involves the cell importing the virus along with a little of the cell membrane. The virus is then transported into a part of the cell called the endosome, which is more acidic than the rest of the cell. This causes the virus to unpack itself and enter the rest of the cell (also known as the cytoplasm).
Replication
Once the virus is inside of the cell cytoplasm, it is able to use the resources of the cell to make copies of itself. According to the Department of Medical Microbiology at the University of Capetown, this involves the virus sending its genetic material (in the form of messenger RNA) to the cell's nucleus, along with proteins that then make more copies of the virus's genetic material as well as the viral proteins. Once the virus has replicated within the cell, it repackages itself in protective proteins and then escapes the cell via a process called budding. This means that viral replication and escape do not cause the cell to die (although over time the virus can exhaust the cell's resources, killing it).
Antigenic Shift
The influenza virus is unique among viruses in that it keeps its genetic material in eight separate segments, which allows for genetic recombination. Genetic recombination is the result of portions of the influenza virus' genome swapping genetic material with other portions of the genome. This causes new strains of the influenza virus to evolve quickly and is one reason why it is difficult for the human body to develop prolonged immunity to viral infections. When the influenza virus RNA recombines, it causes the virus to change, which means that the body's immunity to it (either from a vaccine or a previous infection) is no longer as effective.
