All this is written in various places - some in the bible, some in writings of the great exorcists.
Families are haunted for generations by the same spirits. The latest pope has written about the loss of understanding and loss of acknowledgement of the 'spiritual world'. B16 has hit the nail on the head. "Modern' secularists can't grasp anything they can't hear, see, smell or touch. Nevermind that dogs can see in the infra-red, raptors can see in the ultra-violet and any outdoor animal has a superior sense of smell to indoor human noses.
The world of the spirit is largely ignored by all since they dont see any of its direct influence. Thank the lord there are still those who know evil when they see it and know what to do about it...