The Host is based on a relationship between two people united by costumes, languages and complicity. This confortable equilibrium is altered by an external force, something they cannot touch or identify. Curiosity pushes them out of their confort zone. The desire for something new triggers different reactions, impulses and fears, something they never experienced before. From that moment, nothing will be the same.
The desire to restablish unity, leeds the host and the parasite to adapt to eachother like in a symbiosis.