A person may see, hear, taste, smell, or feel something that is not there. Hallucinations can be frightening for the person experiencing them and for those with them. If you are with someone who is having hallucinations, keep calm and seek medical care as soon as possible. There are many reasons for hallucinations. Many times the cause is easily detected and treated.
Illusions are different from hallucinations.
Illusions involve misinterpreting something that is real. Everyone experiences illusions at some time. They are generally not a cause for concern. People with only partial vision or partial hearing often experience illusions. This is because human brains have a natural tendency to fill in missing gaps.