I quite frequently see people screaming abuse at my friends and I at the open air preaching, but that's because they've just come out of the pub, lubricated with lager and whisky. Half of them are so drunk they can hardly stand. Are these people demon possessed, or merely intoxicated, aggressive and needled by what they see as a group of well-dressed strangers telling them they're sinners and need to repent?
Also, you can come across people who're perfectly willing to talk to you in an apparently calm manner about your beliefs, and use subtle arguments and devious lies to try and undermine the truth. Some of these people, when you refuse to accept their arguments, can get quite angry.
Then you get the people who are violently opposed to the truth and are aggressive and abusive from the outset, without the 'excuse' of being soused.
I can't say I can rule out modern demon possession, and I've spoken to many Christians who think that psychotic behaviour and violent insanity could be demon possession. That could be true... but I have my doubts.
For each for the three types of person mentioned about, there's a rational explanation for their behaviour. The drunks are, well, drunk. For the people who try to win you over with their arguments are ignorant of the truth, it's very annoying for them that their so-called logical and scientific viewpoint is being rejected. The people who're aggressive from the outset may well've had some bad experience involving Christians, are may've been brought up to despise Christianity because of some of the atrocities which have been done in the name of Christ.
Undoubtedly, the Enemy is at work in all these people, tempting them, poisoning their minds, bringing wrong influences to bear on them.
As to the medical explanation, I find that difficult to accept. Perhaps I'm blinded with science, but as far as I know, the medical people can usually find some abnormality in the brain or some external factor which causes insanity or psychosis. The human brain is a wonderful and complex instrument, but God in His wisdom has designed it with failure built in, possibly to keep us humble in the face of our ultimate fragility. It has narrow limitations, it's physically vulnerable, it can be tricked and tweaked, it's prey to many diseases.
Given that... when someone succumbs to insanity and guns down the congregation in a church, can't this be contributed to years of lingering resentment against religion, then some mental safety-valve failing to operate, resulting in an irrational, violent response? It can't be denying that the root cause of all these incidents is sin, the violent nature of Adam's race, the lust for destruction. So, Satan is responsible for murder, and the many afflictions and diseases were suffer are the result of sin. But, demons?
Also...
She just happened to have more of the physical manifestations of demon activity...dark circles under the eyes, seductive in an evil way, way into devil rock music and musicians. I know that sounds corny and cliche', but I'm convinced there is truth that those kinds of outward signs are not to be ignored and may signify demon activiy.
I just don't know about that,
Jethro Bodine. Say that there are in fact demon possessed persons at large, can appearance be an accurate guide at all? I don't want to be pass remarkable, but some of my dear brethren... I really wouldn't like to meet them in a dark alley if I didn't know them! I look pretty rough myself most of the time.
discernment is inevitable. Once you are God's child, I believe you would always know when evil spirits are around.
I see you are a troublemaker. Just talk about Jesus to them and you'd see them manifest. Some would run away. 
