David Shackle was a doctor at an inner-city hospital in San Francisco.
The conditions he regularly witnessed gave him a pessimistic attitude about life, which was encouraged by Joanna Harris. She wanted to turn him into a vampire so he would help with her research.
When he appeared most vulnerable, Harris offered to make him a vampire. He refused, but later changed his mind, and allowed Slake to turn him.
When the other vampires attacked the Eighth Doctor, Shackle did not join them, still wondering if he had made the right decision. This meant he was the only vampire to survive in San Francisco. (PROSE: Vampire Science)