Residual hauntings are not really "hauntings" in the usual sense, because there is no spiritual consciousness attached to the ghostly happenings. Buildings and land can absorb emotional life energy from the beings that live there, and when atmospheric conditions are right, those energies can be re-released, causing all sorts of strange phenomena. You may see apparitions, hear voices, music, or footsteps. But unlike traditional hauntings, there will be no interaction between you and the "ghost". There is no ghost really, just a series of events "replaying" themselves, much like watching a movie. Often, spiritual cleansings can eliminate this type of haunting, since there is no actual entity causing them.
There is alot of debate about what Poltergeists really are. "Poltergeist" comes from the German words, "Poltern" - which means "to rumble", and "geist" - which means "ghost". In other words, a poltergeist is a noisy, troublesome ghost. My personal opinion is that poltergeists are human, dark spirits. They enjoy causing trouble, scaring people, and generally making our lives miserable! These types of ghosts can be extremely difficult to remove, simply because they are strong. Think about this for a moment: if I asked you to levitate the chair you are sitting in without using your body, would you be able to do it? Probably not. Yet, that is exactly what Poltergeists specialize in. They have learned how to use to powers of their minds - their energy - and to draw from the energy around them (including our energy!) to move physical objects. Not an easy feat! Furthermore, they enjoy this type of activity and have no intention of stopping it just because we want them to. That makes it even more difficult to remove a spirit of this type.
Poltergeists tend to draw much of their strength from our emotions. It is very common to have poltergeist activity when you have a child entering (or already in) puberty. These children give off massive amounts of emotional energy, which the ghost can use to its advantage. Likewise, they can feed off of OUR emotional energy. It is highly advisable to remain calm when dealing with a poltergeist. Getting angry or extremely frightened only makes matters worse (easier said than done, I know). I also advise that you do not try to remove a poltergeist yourself unless you are very experienced in spiritual matters.
There are certain spirits that will latch onto the living, and actually drain a person's life energy. (usually a very gradual process) Like a tick, these spirits thrive at their hosts expense. There are many types of parasitic entities (sometimes called Spirit Attachments). Some are human, some are not. But all of them are equally dangerous. So, what happens if a parasitic entity latches onto you? Do you just drop dead because they sucked all of your energy away? No, not at all. But you may start feeling alot of symptoms such as extreme fatigue and weakness, dizziness, clouded thoughts, lack of motivation and will, tendencies toward addictive substances/activities, poor sleep, nightmares, etc. (Note: just because you have some of these symptoms does not automatically mean that you have a parasitic entity. Get checked by a doctor immediately, it could very well be an illness or physical condition that is causing the symptoms.) Having said that, if a parasitic entity hangs on long enough, draining your energy away, illness will likely result, because the body no longer has the strength to fight it off. In some cases you can remove parasitic entities yourself, but sometimes you may need the help of a qualified spiritual practitioner.