I believe if you believe in something or, let's take for instance you go into a demonic location and you have a lot of fear: that fear within your body, within your mind, is like chum in water attracting a shark. They feed off our emotions. These demons also will influence and inflict malice into a living person because that's how they feed.
After dealing with so much dark energy and investigating violent people for my job, I value creatures with pure souls - animals, especially dogs. I believe a higher power/god created these creatures to give us balance from people who can be truly evil. I am an active voice and supporter against animal cruelty.
I've been on investigations where a spirit is channeling through me, and I have extreme changes in my emotions - anger, sadness, confusion. Then I begin seeing visions that are not mine. They are theirs. There is no trace of time. My body goes stiff, numb, cold. Then, when the spirit leaves, I can barely stand and speak.