To begin understanding the nature of parasite spirits, those that seem to attach themselves to and draw energy from individuals, consider the nature of spirits in general. First, spirits aren’t from around here: they are either tourists or invited guest that didn’t know when to go home. Either way, the material, physical universe is not their natural habitat.
Spirits, whether invited (summoned) or attracted to a place, a thing, or a living person, need energy to manifest in the physical world. Summoners, alone or in groups, raise the level of energy using it to attract and manifest a spirit. Those tied to a location are usually attracted by any event causing a sudden increase in emotions or a sharping of the focus of attention, either of which increases the energy level. The key element is the energy state of living things, it doesn’t have to be human, but must be alive.
With volumes of books on the subject, how spirits manifest is covered pretty extensively. Generally speaking, when summoned or attracted by events, spirits are bound or anchored by the conditions that bring them here. Our concern, are those spirits that have become unanchored. Summoned spirits can be unbound by accident or, in some cases, by design of the summoners. Location spirits can be unbound by the alteration or destruction of their domain.
Most will simply return home when the energy dissipates. Some will find other places with similar energies to move to. Of those that stay many will drift around collecting enough energy to sustain themselves, rarely interacting with the physical world around them to any noticeable degree. Usually there is no harmful intent and they pretty much leave us alone.
Parasites are like junkies; they’re hooked on the taste of energy. They are gluttons gorging themselves at our expense. The more they feed, the more they need.
Most siphon off your energy in a particular way. Whether by choice or by limitation, spirits tap specific energy centers (Chakra) or emotional centers. Over time, by manipulating the way energy flows through you, or by tapping strong emotional responses, a parasite can drain you dry. Given enough time a parasite’s drain can lead to physical illness or an emotional crisis just like physical parasites do.
Once they learn their way around spirits can manipulate your energy to reinforce the mode of energy they prefer. How you feel influences your decisions; the results of your choices influence how you feel. A well known pair, the Succubus and her brother the Incubus, live off sexual energy. To increase the harvest they try to keep lust high on the host’s priority list. It’s usually a lot more subtle than that, but you get the idea. You can become fixated on what the spirit needs your energy type and level to be and as soon as it gets its fill, suddenly you’re drained. It can look a lot like a manic/depressive cycle, but it isn’t and it happens over a relatively short space of time.
That influence is what makes a parasite spirit really dangerous. It is also what makes them vulnerable. Once the spirit and the influence are identified removing them entails working around the influence to change the energy flow. It gets more difficult the longer the spirit has been with the host. Essentially, however, the key to ridding yourself of one of these nasties is an awareness of the spirit’s interest, and a willingness to push it off, repeatedly in some cases.