***Spoilers Alert*** Other than the demonic-like parallels in The Host, it's magnificently surprising that the movie writers got the concept of the human host soul right (notice that I didn't write "human host's soul").
It's been said very often that people have a right to their body, specifying a type of ownership of one's body by the self. This is a modern concept called Cartisian dualism. Someone's body and concept of self are said to be two separate entities. (This is very evident in the abortion and same-sex "marriage" debates.)
The Christian concept of self is the integration of body and soul. Pope John Paul the Great summarized this concept best by referring to a person as an embodied soul.
In The Host, the alien "soul" who was really an embodied soul, not just spirit, seemed to be in competition with the hosting embodied soul. This was evident by the way that the main character host was able to physically overrun the alien symbiont by force of will.
Fr. Barron further explains the relationship of the soul to the body ("Faith Seeks Understanding Pt. 12: What is the relationship of the soul to the body?"):
Here's The Host Part I.