Spirits. Just what are they and how do we deal with them?

I was wondering about spirits and how do they come about entering into a persons life?

How does people even know when they have one?

I dont believe anyone with the Holy Ghost can be anywhere near demon posssessed. But what about Oppressed? Just what is oppression?

This is not something I have heard any preaching on but at times a preacher will refer to spirits yet never go on to explain what he means.

Do people visiting a church open those spirits up to people in the church who may have been struggling with there walk with God?

I find it hard to believe a spirit can jump off of someone onto another person. Am I really wrong in thinking this is what Ive heard others talk about?

Thought this subject might open some interesting discussion with alot of good opinions and knowledge.