When a man dies, his spirit can either be taken on another plane of existence where he comes into new state of awareness, with a new body or he can be left on the Earth to wander as a spirit or a living form of disembodied energy.
These wandering spirits roam without realizing they're dead. They move in a state repeating the same things over and over again as if they're living until they're "awakened"..either by another spirit or summoned by a human. For example, if a man and his family is killed in their home..the trauma of the act, the blood being spilled and the raging power of the emotions conjured before death marks that as a resting place for the spirits of those fallen... especially if they have no light.
Again, most of these spirits are unaware of their own death and roam harmlessly. However, those that are AWARE may not take kindly to your presence since you're living and knowing that they're dead will attempt to interact with you with adverse effects by principally trying to possess your body. Symptoms of this physically is random nose bleeds, sudden convulsions and shivering, headaches, constant lethargy, rough sneezing and altered mental states.