The stillborn child
The stillborn child at the novel's onset is a symbol for a few things. First of all, it means the end of a season of confusion following the mother's violent rape. The death is also a natural inversion of motherhood symbolism, such that it symbolizes the sureness of death instead of the abstract strangeness of life. She does not give her life for her child, but survives the child's short existence. The painful symbol is also a comment on the zombie motif, because this novel is set in a zombie apocalypse which also represents the duality of life and death, but in an unnatural way.
Subject Zero
The unholy zombies have risen from the dead in some way other than traditional concepts of resurrection. The quest for Subject Zero is therefore a unique reference to the messiah figure, because Subject Zero is the "firstborn of the dead," so to speak. By finding the center of the epidemic, they hope to discover the secret to surviving this horrible apocalypse. The quest takes a lot of forms, so perhaps it can be thought of as a "holy grail"-type quest among a cast of heroes.
The Republic of Texas
The devolution of world governments in the post-apocalypse is symbolized by the institution of the Republic of Texas. Texas is a good symbol for this because, instead of secession, there is a popular idea in Texas that because of the legal documents pertaining to Texas's annexation, the state can simply nationalize in the case that the federation devolves. The new government is also a good symbol for post-apocalyptic order because Texas is well-known to the politics of life among chaos; this will be a second Wild West.
Amy as archetype
Amy is an archetypal savior because she is legendary. She is literally prophesied as the hero of the novel early on. We see her journey toward becoming ready for the kinds of skills and sacrifices it will take for her to survive and succeed. We also see an archetypal foil in Tim who becomes "Subject Zero," the anti-messiah figure. Unsurprisingly, Tim is absolutely obsessed with defeating Amy and crushing the human hope of survival. Amy becomes practically deified in the denouement when she succeeds.
The remnant
A symbolic remnant survives this monstrous apocalypse. They are a symbol for time and new life. The novel leaves the reader in the dark about whether there is any hope for the human race in light of this terrible infection pandemic. The remnant is a symbol of the highest order—as a motif, it refers to apocalyptic religious literature, like the major prophets. Ezekiel, Jeremiah, and Isaiah all prophecy that a select few will survive the apocalyptic wrath of God in the end days. The remnant is a signal that perhaps the novelist's own wrath has abated.
