What metaphors are used in Pretties by Scott Westerfeld?

Dr. Cable approaches Tally to ask her to join the Special Circumstances. From the recent events Dr. Cable apparently started seeing potential in her. Tally refuses and accuses Dr. Cable of the cruelty of what she and the Special Circumstances are doing to people, making them forget themselves. Dr. Cable argues with the example of Rusties who destroyed themselves and almost all life resources and justifies that what they are doing by altering the humanity is preventing that from happening again. She believes that humans are a plague, a disease, a cancer on the body.