I know something about hallucinations! //ShotdeadbecauseIamone
Hallucinations can appear for multiple reason, human-caused, supernatural, and natural.
If someone is on drugs, of course they will see them, along with second-hand inhale of it. Then, of course, there's the other way like hitting your head just before you go to bed, even reading stories sometimes influences it.
Supernatural, if you are a skeptic, I'd not read this- but supernaturals such as spiritual energy can also cause hallucinations, both in reality and the real world. This is sometimes how they show their presence in our worlds. They can also do this is their energy is negative, like the sixth floor of an abandoned hospital, or a place where someone innocent was murdered, etc.
Then, there's natural. Sometimes the human mind just does it often, and as you age, you are more likely to be prone to this.
And there there's me, slipping in to your dream realm at night to give you nightmares. //shotdead xD