For some game purposes. I think common sense should apply here. Remember this quote from alter self? "Any part of the body or piece of equipment that is separated from the whole reverts to its true form." Now with a rat swarm, you're 300 rats. 299 rats are therefore seperated... thats a whole lot of body reverting.
Plus, I think its a little over the top that a druid should be able to become immune to all spells except those that effect an area, in addition to taking half damage from most weapons...
Plus, I think its a little over the top that a druid should be able to become immune to all spells except those that effect an area, in addition to taking half damage from most weapons...