I am of the same mind on this point.
Basic training should include psychic hygiene, but it’s rarely mentioned. It’s rare for a grimoire to say ‘oh and before you dive in, it’s beyond the scope of this book but go ahead and learn how to shield, ward, cleanse and banish’.
Here in the unofficial tutorials thread, detecting impostors is top of the list, but there’s nothing on parasites specifically… I think it would be a good idea to post some there 
I will look into it. If you know of a tutorial can you add it? that would be cool.
In this case, it’s not clear if the OP did anything to attract this yet.