Influenza is caused by several members of Orthomyxovirus, a family of viruses. Of interest are the Influenza A virus (that infects humans, pigs, horses and birds), the Influenza B virus (that infects humans and seals) and the Influenza C virus (that infects humans and pigs). Of these three, Influenzavirus A is the most virulent and regularly causes pandemics. Influenza viruses are not ‘stable’ and have the tendency to change.
You should take your annual flu jab and be reasonably protected. Or you could take the chance and hope your immune system is strong enough to let you survive the infection. However, if that immune system is compromized you may need hospitalization and be treated with antiviral drugs like Tamiflu (oseltamivir) or Relenza (zanamivir).
But the virus fights back and a slightly mutated or adapted version is now dodging the antiviral Tamiflu. Scientists think that resistance to Relenza (zanamivir) is only a matter of time.