Mongooses have thick coats that protect them from short-fanged snakes and they are extremely agile. But the main reason is that they are highly resistant to snake venom. One of the main constituents of the venom is a toxin that mimics the shape of the neurotransmitter acetylcholine. If you are bitten, the venom attaches to the receptor sites on your nerve cells and blocks the normal action of acetylcholine. This causes numbness and paralysis. Mongooses have a different molecular shape to their receptor site so that acetylcholine still binds to it correctly but snake venom does not.