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See, now I'm confused. I bought the Richard Laymon book and although it involves snake-like creatures invading people's bodies and making them do unmentionable things to each other, it is still different to the story I read.
The story I read was a SHORT story, and the creatures, as you say, appeared overnight. In the Laymon book, the creatures don't actually seem to feature that heavily; it is more about some serial killer bloke.
It is definitely a different story and it is still bugging me lol!