I'm gonna say that Carlisle was infected, he infected Edward, Esme, Rosalie, and Emmett. Jasper was infected by Maria, and Alice by that orderly dude from the asylum. Bella, would later be infected by Edward, after giving birth to their half human/half zombie child, Renesmee.
When Renesmee is born, she rips her way out of Bella's body, so Bella has a gaping bloody hole in her, along with a multitude of broken and snapped bones. Edward have to infect her before she dies.
So, if you noticed, Edward's cheek is decaying as is most of them's. Scratches cover their bodies as well as yellowish bite marks. Blood splatter and hanging flesh. Not for the faint of heart. Who's looks best? Worst? What's the difference?