Every thoughtform can be imagined, from an armchair to an Angel. It is a conscious act which is immediate though not everyone can do this. Hallucination and Meditative visions are more complex. They are both demonstrate an altered State of Consciousness. Since man first discovered drugs to hallucinate they have been used throughout the ages in religious ceremonies, by drug addicts and milder drugs for recreation. It is well known that any drug can cause harm in some way.
What you are talking about is hallucination due to illness. Similarly to the drug induced hallucination, you do not have control over this. Certain types of illness can trigger these visions which exist on all levels of Consciousness available to everyone. Heavenly visions or demonic visions can be viewed. As a person gets more ‘out of’ his physical body due to a very serious illness then we come into the realms of near death Experiences.
Meditative visions are an entirely different matter.