IF that be the case...
With a third of them siding w satan,
why don't they just have their way with everyone then?
Because demons don't have such absolute power! The majority of their power comes from deception, leading people into various sins, and they can only do that to the degree that people accept their tempting whispers.
That's why God gave us our consciences!
And that's why God told us to take every thought captive.
And that's why God gave the Law of Moses, ie. if our consciences failed, the threat of the hang man might be sufficient to restrain a rebellious person from murder.
They could easily wipe out the hospitals; bam! cancer for all.
Faster yet, heart failure for everyone, on the house.
There is absolutely no doubt that demons are the cause of many sicknesses, but to succeed they still have to cope with man's God given immune system.
Demons are opportunists, and like opportunist burglars always on the lookout for open doors and windows. That's what Peter meant-
1Pet5v8Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour.
A prowling lion doesn't go for the strong, but the weak and vulnerable, likewise demons.
I would add that not all demons which afflict our lives are down to personal sin.
Many people suffer various levels of demonisation because of some ancestral sin.
Demons can also gain access via trauma, either physical, mental, or spiritual.
Examples-
Spirit of death.
I recall one of the earliest deliverances I did was a lady that came to our house group in the early 80s. By chance, she was the first person to come on the scene of a car crash, and being a nurse she had to deal with it at night in the dark and on her own.
In the week that followed she was having sleepless nights and asked us to pray for her. I discerned that a spirit of death from that crash site had entered her as she fearfully crawled into the wrecked car. We commanded it to leave her, and that immediately ended the problem.
Spirit of grief.
After several years of suffering, my dad died of cancer when I was in my late 20s. For years afterwards I couldn't even talk about him without grief and tears flooding out. No matter how I tried, I had no control and it was very embarrassing, until one day I realised it was far more than just plain natural grief.
I then commanded a spirit of grief to get out of me, and once it departed, I became free to enjoy talking about my dad without all the tears.
Spirit of Mormon.
I've mentioned elsewhere of discerning a spirit of Mormon in a woman. She vehemently denied ever having any connection to the Mormons, but as we proceeded, she suddenly recalled a childhood conversation with her mother. Apparently, her great great..................grandfather had travelled out to Salt Lake with Brigham Young in 1846!
The demon of religious deception, starting in that man had passed down through each generation and each time working to stifle their relationship with the Lord. Again we cast it out and set her free.