These astral fragments need etheric energy (life-force) to hold them together or they will eventually break up into astral dust and be reabsorbed. The only place they can get enough energy to survive is from a person or animal but they certainly seem to prefer a human host.
When they find someone suitable often with similar emotional characteristics to itself, the entity will try to get into the person’s energy and live there. This will drain life-force (etheric energy) from the person and also impose emotions on them (astral energy).
Very large fragments with enough etheric energy may survive without a host and these may live in houses or public places. These large entities are usually referred to as ghosts. They may attach to a person with a cord of astral/etheric energy while remaining in these places and then affect the person similarly to a smaller fragment inside their energy.
Other entities that can parasitize people include nature spirits, elementals and others which are not so easily categorized.
Entities are actually quite a problem at present with most humans being the home for one or more. With our senses being so dull due to such crystallized astral and weak etheric bodies we can only feel the effects of the entity. Often these effects will be lack of energy, depression, anxiety and all types of other negative emotions. We assume that these emotions are our own and therefore have no idea that an entity is causing such problems. Actually the entity isn't the sole cause of the problems. To attract an entity you must first resonate with it to some degree. So you had some feelings of loneliness or whatever and so did the entity. This is what attracted it and so your initial negative emotions are amplified by the entity and it also drains your etheric energy making it harder to stay on top of such negative emotions.
An entity can only get into your energy if there is a weak spot or hole in your aura. Most people have some weak spots which become worse when under pressure or emotional stress. It is due to ego structure and astral crystallization that weak spots occur. In fact the very nature of Astral Substance attracts entities. When it comes to Astral Substance, like attracts like so you have a situation where different types of Astral Substance pool together and keep on attracting more. This is how the