i think this song has to do with the singer going thru a break-up. in the beginning, the relationship he had with the other person was what he felt like in the beginning of the song: parasite and host. but as the song develops (like the relationship) he begins to like the thought of having his other around. by the time he is cleansed of her/him, and then at the end he is wondering where they went, which shows he really wanted them around after all.
i think this song has to do with the singer going thru a break-up. in the beginning, the relationship he had with the other person was what he felt like in the beginning of the song: parasite and host. but as the song develops (like the relationship) he begins to like the thought of having his other around. by the time he is cleansed of her/him, and then at the end he is wondering where they went, which shows he really wanted them around after all.