What is Role of HA in the biology of influenza virus and how this protein influe - (Aug/20/2013 )

Hi Please help with the question below 

What is Role of HA in the biology of influenza virus and how this protein influences the nature of disease caused .

I have understood the role of HA (which is a glycoprotein and is important in influenza replication).... ie receptor binding and membrane fusion: Ha precursor Hao has 2 sub units: Ha1 ans Ha2 where it contains receptor binding domain and the second contains fusion peptide.

but Im having difficulty understanding how this effects the nature of disease caused.

I think its to with antigenic drift and antigenic drift but does that not just give rise to new strains? is that same thing as nature of disease caused?

Please help with a description as I have read many explanation but having difficultly understanding.

Any help would be much appreciated.

Thank you 

Your second thought is the one they are probably getting at - though I think you mean drift and shift (rather than drift and drift) - think susceptibility  and immunity with regards to the HA and NA strains.