Let’s look at a few cases of demon-possession in the New King James version of the Bible to seek to answer your questions.

In Mark 9:17-18 we read, “Then one of the crowd answered and said, ‘Teacher, I brought You my son, who has a MUTE SPIRIT. And wherever it seized him, it throws him down; he foams at the mouth, gnashes his teeth, and become rigid. So I spoke to Your disciples, that they should cast it out, but they could not.’” As I read through the various examples of demon-possession in the New Testament, it is often limited to ONE INDIVIDUAL in a family as we see here. So, even though the “bad spirit was brought into the home” through the man’s son, no one else was possessed. That’s not to say they weren’t affected, for imagine the heartache the father experienced as he saw his son suffering untold misery as the bad spirit took away his ability to speak and hear (see verse 25), caused violent seizures, and threw him into fire and water at times (verse 22). Add to this the fact that his son had been plagued with this spirit “from childhood” (verse 21). Did the son know he was possessed? We can’t say for sure because we aren’t told, but it seems likely the father tried to inform his son at times when he had the ability to understand. Let me add that today there may be similar cases to this one and yet people aren’t as likely to believe it is a case of demon-possession. The violent seizures described in this account have been passed off as a severe case of epilepsy.

Now let’s look at Mark 5:2-5: “And when He (Jesus) had come out of the boat, immediately there met Him out of the tombs a man with an UNCLEAN SPIRIT, who his DWELLING AMONG THE TOMBS; and no one could bind him, not even with chains, because he had often been bound with shackles and chains. And the chains had been pulled apart by him, and the shackles broken in pieces; neither could anyone tame him. And always, night and day, he was in the mountains and in the tombs, CRYING OUT AND CUTTING HIMSELF WITH STONES.” This is another severe case of demon-possession and once again we see that it is ONE MAN who is the victim of an “unclean spirit.” We don’t know if he had a family or a home nearby, so it’s impossible to tell if this “bad spirit” was brought into a home. It seems likely he was a “grown man” (and not a young adult) and in this terrible condition he chose to dwell alone (among the tombs and mountains). I have only had one occasion to be with a man that was, in my opinion, demon-possessed and he always wanted to be alone. I don’t believe he was aware of the demon, and it’s quite possible the man in our story was not aware of the demon as well, for later after Jesus cast the unclean spirit out (verses 9-13) we read that the man was “sitting and clothed and IN HIS RIGHT MIND.” While he was possessed, he was always walking about and trying to hurt himself, so it seems he had lost all control of his thoughts and actions. The demon had FULL CONTROL of him and he was UNAWARE of what was happening. Again, there have been cases today of men and women with these same symptoms of: 1) Isolation, 2) Violence, 3) Supernatural Strength, 4) Depression, and 5) Suicidal tendencies. Yet men are reluctant to consider them cases of demon-possession and will usually diagnose them as criminally insane.

In Acts 16:16 we have another case of an individual being possessed. It says, “Now it happened, as we went to prayer, that a certain slave girl POSSESSED WITH A SPIRIT OF DIVINATION met us, who brought her masters much profit by fortune-telling.” Here was a case of possession where the only “symptom” was having the ability to “foretell the future.” My wife and I just spent 3 days in Sedona, Arizona and we were saddened by the many stores and shops that had signs advertising “seers,” “palm readers,” “fortune tellers,” and “crystal gazers” who could tell you your future. Perhaps some of these are frauds, but some may be, like this slave girl with a “spirit of divination,” possessed with a spirit. Do they realize they are possessed with a demon? I doubt it, for we actually overheard a conversation where people were discussing the “special gifts” that these people have been endowed with. No doubt those with a “spirit of divination” think they are very special. Yet those poor souls (the slave girl in Acts 16 and some of the people foretelling the future in Sedona), are victims of malicious spirits whose purpose is to keep them blinded to their need of Christ and the salvation He offers them through His sufferings and death on the cross of Calvary.  (469.3)  (DO)