Ahem, please consider that a swarm is a multitude of creatures. Also, most swarms are composed of "diminuitive" sized beings (with the one exception, a "rat swarm") , which are impossible for a druid to wild shape into at any level.
Of course the basic druid (non-Eberron ) can only shape into creatures of the animal subtype (and later elementals) in the first place, which most vermin are definitely not.
And the, description of the "Wild Shape" power states :
"At 5th level, the druid gains the ability to turn herself into any small or medium animal and back again once a day. "
Animal , as in singular being. Not "animals". And the reference to the actual animal subtype is in a later sentence, so nothing to be confused about at this particular point. So, IMHO, no rat-swarms. And no sending a single rat to safety (and survival) and fight on/delay with the rest.