The devil roams this world like a roaring lion seeking whom he may devour, his demons seek to express themselves through people and animals. Whilst Christians cannot be possessed, we can be oppressed, e.g. spirits of infirmity, spirits of depression and anxiety,etc. I have personally never been led to do deliverance on a non believer, simply because they will just return and will be worse than before. The person needs to desire to be set free or they will just open the door again to demonic activity. Many of us come into our Christian walk bringing with us baggage from the past,meaning curses (generation curses, evil pronounced) occult practices (zodiac signs, fortune telling,yoga,eastern meditation, mind over matter.etc), cult beliefs (believing in a figure or person who is not Jesus-God). Often we a possible sign of demonic activity operating in ones life could be that you often feel no matter how hard you try things never seem to go right for you or perhaps hereditary diseases etc. Most of us need to be set free from demons, but what surprises me is that very little is taught about it in the mainstream. I have seen God do amazing things in the lives of people who have been oppressed. One lady was going to be sent to a mental home, my friends took her through deliverance and today she is well and happily married. Another lady had an injury to her wrist, the Lord said to me cast the demon out and she will be healed, the demon manifested in her by trying to get her to bite me,but God touched her in an amazing way. Often healing (physical or inner) goes hand in hand with deliverance. Here is a clip of a AA Allen who was used mightily of God in the area of healing and deliverance.