A symbiont enters the host's body
A host was a lifeform affected to some degree by an invasive or exotic entity. A host and symbiont relationship were the most common form. In such a situation, a symbiont was an organism that became part of the host's resources – including (but not limited to) mind, thoughts, emotions, or body. In some situations, the host's resources were shared and the host considered the relationship unwanted. This type of relationship was considered parasitic. Both of these types of these relationships required a host that the invasive entity deemed appealing in some way.
In 2266, James T. Kirk implored Doctor Leonard McCoy to find a way to kill the Denevan neural parasite without also killing the host. He later recorded a captain's log denoting his reluctance to kill the Denevans despite having a prerogative to do so rather than risk the parasites spreading further. (TOS: "Operation -- Annihilate!")