Swine flu is unlike other flus for several reasons.  First, it isn't a flu for which a vaccine has been developed.  Secondly, it's unique in that it has 3 different strains of flus.  Also, it has the potential to recombine and come back in a deadlier form like the pandemic of l918.  Although it is said to be missing an enzyme, if it combined with another strain this could change.  There is a lot that science does not know about this particular flu and how and whether it will mutate.

It has been suggested that some segments or age cohorts of the population might have some immunity to this flu but at the same time I believe it has been prudent on the part of the government to play it safe as it appears to be a fairly contagious bug coming at an odd time of year.