Do ghosts and demons exist? If so, can demons hurt us and how can we protect ourselves from demons?
Ghosts and demons do exist, but perhaps not as the world thinks they are.
Ghosts, supposedly, are the souls of the dead. The Church does not say much about them as such though, and for good reasons, since the souls of the saints are in heaven, and would only come to visit us when they are sent by God on a mission. The souls of those in purgatory are too busy getting purified, and it seems that the only time they come to us were to certain holy persons so that the truth of purgatory may be made known to us. And the souls in hell, well, they are too busy being tortured. Some scholars theorize that the only souls that would fit in the typical understanding of ghosts would have to be those in hell, since the other ones would be messengers from God, not just some wandering ghosts. But if they do exist, they are not capable of harming us; unless somehow we see them and allow ourselves to be afraid.
But I personally suspect that what most people claim to be ghosts are really demons disguised as ghosts to harass us.
As to demons, they are the fallen angels, the minions of Satan. These have real spiritual powers and could cause real harm. However, we do not need to be afraid of them.
First, because they are creatures and so their powers are nothing compared to God’s. And as such, they have already been defeated by Christ. As long as we remember that we are God’s children, they cannot harm us and we have nothing to be afraid of. In fact, we can even cast them out in the name of Jesus, if we believe with confidence.