When a person sneezes, the droplets can spread up to 150 feet a second. When the moisture from the droplets evaporates, it leaves behind thousands of influenza germs. These germs may be inhaled as they float through the air or be picked up on your hands from contaminated surfaces. The next time you get the same flu, your body would recognize the virus and produce antibodies to fight off the flu. The problem is, these antibodies will not protect you if the influenza virus has changed its structure.
Viruses can change their structure during the replicating process .
When the virus makes the new RNA , it makes an exact copy of itself, like a mirror image. Influenza virus does not have a system to notice or correct for errors during this process. These errors can make changes to the new virus that are different enough that your immune system cannot recognize it. If the immune system does not recognize it, it will not be able to produce antibodies against this new virus. Without antibodies, the virus can go about its business of infecting your cells.