When Vampire are hummans and someone bites them how come they dont just die (the person whos bein bit)?

Because if they just bite the venom has time to spread through the veins making them turn but if Vampire are hunting then they suck the blood draining it from the body therefore killing them. Thats why the Cullens are always worried that they dont have the restraint to stop once they taste human blood but carlilse did