For spirits to function on this earth, be it an evil one or otherwise, it needs to take possession of a mortal. This is why it is reckless for us to live out lives mindlessly. Spirits are always looking for who to possess and whichever body they find empty is vulnerable.
When it comes with that of the Holy Spirit it doesn't force itself on anyone. Whoever the Holy Spirit lets Himself into must be a person that offered himself willingly. But it's not the same case with that of the evil spirit.
Matthew 12:45 - Then goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first. Even so shall it be also unto this wicked generation.
The above is the case of someone who was once possessed with an evil spirit that was later cast out. But even if after the evil spirit is being cast out of the person, if the person should not bother about doing something to prevent such encounter to happen again, the spirit will eventually return. And the painful part is that it won't just return alone, it will return with seven more spirits that are even more wicked than itself.
As children of God we are gifted with the Holy Spirit and as long as we let it dwell in us we make ourselves inaccessible to the attacks of the enemy.
The holy spirt is our daily guide against evil spirits or attacks.