Demons:
Demons are like Ticks. In other words they drop off of trees and attach themselves to you. Think of it like this, when you do something that is a mortal sin, they drop off the proverbial hell tree and land on you. This is infestation, then they attach themselves to you because of some sin you tend to commit over and over. This is how they stay with you. This makes them extremely tough to get rid of.
Here is a short list of mortal sins which I have found most common in diabolical cases.
*Sexual sin- Promiscuity, homosexuality, and sodomy. Demons revel in this sin.
*Religious sin- Renunciation of God and Jesus. Satan worship in the form of Wicca, Satanism, or Witchcraft/occult practice.
*Generational Curse- Someone in your family did something very naughty, and it is following the family.
Remember the Seven Deadly Sins, keeping away from them, and living within the ten commandments will get you 80% there.