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. For instance, it is not unusual for elderly people to mistake the hum of an air conditioner for the murmur of voices. After someone dies, it is normal for people who were close to that person to see or hear that individual on occasion. This is generally not a concern unless it becomes distressing.
Grief counseling can often help.
In post-traumatic stress disorder, people have experienced severe traumas such as rape or the events of war. They may also have illusions where they distort reality. For instance, someone with post-traumatic stress disorder might see a police officer's uniform as a military uniform. Hallucinations can be related to certain mental disorders, such as schizophrenia. However, the hallucinations can generally be controlled with proper medication.