What is it about influenza that makes this disease continue to plague humanity?

What is it about influenza that makes this disease continue to plague humanity? Each year, a very large vaccination campaign is waged on a very significant portion of the population to protect us from the new influenza.  How is it that influenza mutates so quickly that we need a new vaccine each year?  And why do we continue to live under the specter of the 1918 strain, which was particularly lethal? 
Read the attached article regarding the development of swine flu vaccines.  How does the situation appear?  Are we protected?  Relative to standard influenza, how big of a concern is swine flu?
APA format, in-text citation, reference include, 1 page ( 2 paragraphs that cover all the main point).

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