Venom

Venom is a kind of matter that while a vampire bites a person, venom will be passed from the vampire's teeth to the person (maybe into the blood vein) then that person starts to transform into a vampire, if his or her blood isn't fully sucked away.

According to Midnight Sun, vampires may 'drip out' some venom from their teeths while unresistable, which happens on Jasper.