I only know of one guy where a rattlesnake crawled in with him while he was sleeping. http://www.riflemanconnors.com/and_the_devil_makes_five.htm Always carry plenty of cigars when you are in rattler country.
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Snakes are pretty smart. They generally can recognize larger predators and instinctively know to stay away. If given a chance most snakes will leave you alone unless you force them to defend themselves.
Snakes seeking warmth and crawling into a sleeping bag, makes for good stories, but I have never heard it from a reliable source. I suspect snakes also know not to curl up next to a sleeping bear.
Ticks and bugs on the other hand seem fearless. I am not sure if repellants work on ticks (I think they do), but they seem to work well for everything else.