'Expatinscotland no, it is not just that they can mutate, even if they don't mutate, the immunity against the virus build up either through having that strain of flu or an immunisation for that strain of flu is short lived and this is the problem.'
Um, no, the immunity conferred by actually having the disease is, for most people (probably excepting those who have compromised immune systems) not 'short-lived'.
That's why people who survived Spanish Flu usually did not get another form of avian flu. In their lives. Indeed some, like my grandmother, appear to have passed immunity onto at least one of her children (via birth or breastfeeding, who knows), as he was once in a study when he managed not to contract Hong Kong flu (a form of avian flu) when the rest of his family did.
Or why older people are not as affected by swine flu. Because in the past, many of them had a strain of swine flu.
Same as with measles, mumps, rubella and other viruses like this.
You only get them once unless you are immuno-supressed/have a faulty immune system.
Same was with rhinoviruses, it's just that there are so many of them, it's impossible to have caught all of them over the course of one lifetime.