Yearly vaccination of humans for influenza viruses is necessary ...

[futuristic]

Yearly vaccination of humans for influenza viruses is necessary because
A) of an increase in immunodeficiency diseases.
B) flu can generate anaphylactic shock.
C) surviving the flu one year exhausts the immune system to nonresponsiveness the second year.
D) rapid mutation in flu viruses alters the surface proteins in infected host cells.
E) flu leads to autoimmune disorders.

[aero]

I remember it was D