While the novel The Host is filled with imagery of a futuristic world and alien body-snatchers, one could easily suggest that the main idea behind the novel speaks to the old adage "Don't judge a book be its cover."
In the novel, two separate "beings" are forced to inhabit one body. Each of the beings, Wanderer and Melanie, have their own preconceived ideas about what the other represents. By being forced to "live" with each other, each comes to see that those of the other "race" are not what they have been told.
Initially, each are fearful and angry at the other based upon what they have been taught and the preconceived ideas they possess. Over the course of the novel, each comes to find out that one cannot "judge a book by its cover."