Symbolism, Imagery, Allegory
Images of pollution are used throughout An Enemy of the People. First, of course, there's the polluted Baths, which have become infested with bacteria. This filth is the literal industrial pollution from the tanneries near the Baths' swampy water source. Ibsen's use of this sort of imagery doesn't stop at the literal, however.
Throughout the play, images of dirty or tainted water are used symbolize moral and societal corruption as well. Early on Hovstad describes the group of corrupt officials that run the town as "The morass that the whole life of our town is built on and is rotting in" (2.79). (By the way, a "morass" is a swampy or boggy area.) Hovstad, who comes from a lower class background, sees the rich and powerful as the true corrupters of the land.
Dr. Stockmann comes to his own conclusion about where the real pollution comes from. He bellows before an angry crowd, "it is the masses, the majority – this infernal compact majority – that poisons the sources of our moral life and infects the ground we stand on" (4.101). By the end of the play, Dr. Stockmann is much more concerned with the pollution he sees caused by the complacent masses than he is by literal dirty water.