Just to be clear the "flu" virus or any virus for that matter does not have teeth,fangs,nor claws.It has a outer membrane with paterns of protiens/chemicals that allow it to pass through the walls of our cells.It then incorprates its dna/rna into our own cells replication cycle to produce more virus particles.What is "mutating" is the pattern of these membrane chemicals.Thier is random variation with each new virus particle,the "winning" combo is the one that our bodies' defenses have never seen before,thus no early immune response is mounted,and infection is rapid,that is all.Viruses don't think,don't plot,they are an exsquisit packet of dna/rna that have evolved:frown: