Can the consciousness of an organism invading a person alter their beliefs to promote greater vulnerability as a host?

This can indeed happen. The presence of a foreign consciousness, and particularly on the part of organisms recognized by the immune system as an invading force, may well have consequences emotionally, and altering the physiology as through the stress response, for example. This can be an alteration of mood state to the negative, but also trigger incorporation of new negative and self-limiting beliefs that can be a significant liability and even a crippling influence causing someone to reach a state of despair and hopelessness, as seen with clinical depression. So this is, at times, an appreciable traumatic event, and at any level will add to the difficulty in making a recovery from illness.