Astral parasites enter the atmosphere through leaking from one world to the next. They pass through electrical outlets, equipment or tools. They invade the rivers, the lakes, the oceans, the animals, the plants, the food supply.
They ooze through, unseen at first and then via clairvoyance they are seen, and via clairsentience they are felt.
Imagine a crack in the door and tiny, tiny ants enter the room. They are not seen yet, but as more and more enter, they become more visible, but by then they have already caused damage.
Astral pests are the same than physical pests. They follow the same pattern. All it takes is a slight crack. More and more of these cracks are happening. It will take a lot of work to try to prevent the leakage because as you block a crack, new cracks are forming because of negative activities.
These parasites consider every living thing as food. They strengthen themselves through sucking out the energy of a living body, be it animal, plant or human.
They may even land on a non-living object and wait for their entry into a living body. For example, at a hotel, they may lay on the sheets and towels, waiting for guests to lay on them or to use them.
They are parasites and they do what parasites do. They wait for the moment to invade the body. They strengthen themselves and they survive just like any other parasite.
How were they unleashed unto humanity? They act like all other opportunists. They saw open doors, open avenues, cracks in the fabric of time and they came after food. They liked what they got and they telepathically communicated their enjoyment to others. And more came.
The idea is to always keep your doors tightly closed, but that would require knowledge of an impending attack, knowledge of an unknown world and control of outside forces both human and non-human.