Venom

Venom is an organic, poisonous fluid that is produced by saliva glands within a vampire's mouth. It serves six purposes:
#As the basis for their physiology.
#As a means to paralyze their prey in pain to prevent them from escaping.
#As a pleasant scent to lure in the vampires' human victims.
#As a toxin poisonous to animals and shape-shifters.
#To repair a torn limb of a vampire.
#And, very occasionally, as a means for vampires to propagate their species. The victim cannot run so the vampire can kill the victim or allow her/him to change.
When a vampire bites a human, the venom will be passed from the vampire's mouth into the person's bloodstream. If the blood contaminated with venom isn't fully sucked away, the human's heart will pump the venom throughout his or her cardiovascular system and start to infect every cell it passes within the host's body. Over the span of 2-3 days, the venom will have saturated all cells within the host, completing his or her transformation into a vampire. The process is known to be slow and painful, described with the feeling of being burned alive.
When a vampire is bitten by another vampire, the venom from the teeth will create a permanent bitemark usually invisible to human eyes, and the pain is supposedly stinging but eventually passing.
By licking the venom on an open wound or the edge of a disembodied limb, the wounds will successfully heal. However, it does not make a lost body part grow out again or reattach torn hair.
Vampires may 'drip out' some venom from their teeth while smelling something irresistible, as shown by Jasper in Midnight Sun.
Real werewolves are the only known species to be completely immune to vampire venom, whether in the painful or transformative departments.The Twilight Saga: The Official Illustrated Guide
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