The way in which influenza replicates
Influenza is a virus with specific viral enzymes. One of its enzymes, RNA dependent RNA polymerase is responsible for the replication of its genome. One of the characteristics of this polymerase is that it is error prone. It works quickly, but a lot of mistakes are made. Some of these mistakes change the outer appearance of the virus which in turn causes the viral particle to become invisibile to the human body.
Another thing the viruses of the influenza type do is exchanging parts of its genome with other influenza viruses upon simultaneous infection of a host cell