The term "demon" is a generic term that encompasses devils (fallen angels) and unclean spirits. Many believe that some "unclean spirits" are spirits that originated here on earth. Many believe that they are the disembodied Nephilim of
Genesis 6 who drowned in the flood. Others believe that these are spirits of the "unpurified" or "impure" dead who are essentially damned and desire to haunt and/or torment the living with fear and various maladies as they roam the earthly plane. Some believe that these are both. These are known as "dybbuks" in Judaism and various rabbinical writings.
Unclean spirits desire to inhabit human bodies more than fallen angels do. This is because they once had bodies of their own. They desire to "live out" their lusts and desires through their host, ever enticing them into deeper and deeper levels of various sins and unhealthy habits... or even heinous acts. Some unclean spirits are jealous of the living and seek to bring sickness and various maladies to inflict suffering upon the living. Either way, be the possession a fallen angel or an unclean spirit, exorcism is the only remedy.