Replying...
Intro. It's about the so-called problem. At the beginning of the first book, it has existed for a good 50 years; after their death, some people return to the world of the living as spirits. Usually they are either criminals, people who did not die of natural causes, or both. The spirits are bound to a source that prevents them from disappearing completely into the afterlife. In order to render this source harmless and thus neutralize the visitor, it is "sealed", i.e. covered or mixed with an iron object. They are made of ectoplasm, which is practically the immortal soul of man. When touching the substance, a living person is seized by the spirit sickness. This is manifested by blue tarnished and swelling body parts. If he is not treated quickly enough with adrenaline injections, the afflicted person dies. There are the gifts to hear, see or feel spirits. Each gift can be of varying severity.

Lockwood

@Arianna