Some people are more susceptible to spiritual infections (obsession and possession) than others. There appear to be four broad categories.
The first category includes people who are more “psychic” than others.
The second category includes schizophrenics, and people with other mental illnesses that can cause psychosis. (This is NOT to imply that all mentally ill patients have spiritual infections. The vast majority of the mentally ill have distorted thought processes because of “ordinary” psychiatric disorders.)
The third category includes people who have damaged their auras through bad lifestyle choices and “bad attitudes.”
The fourth category includes people who were infected because of severe abuse at the hands of an abuser who was him/herself previously infected with an evil spirit.
Of course, some poor souls fall into a “mixed category.” Imagine a psychic schizophrenic who is also a criminal! A vast majority of those in the psychic categories never have a problem with spiritual infection, but they are still more prone to infections with alien intelligences than people who are not psychically “gifted.”