'Chimera' (1955) by Mira Grant

'Chimera' is the third and final book in Mira Grant’s Parasitology trilogy, concluding the story of a world upended by the creation of genetically engineered tapeworms designed to improve human health. Instead, these parasites have caused a global catastrophe, turning many hosts into violent, mindless creatures as the worms take over their bodies.

Sal Mitchell, the protagonist, continues to grapple with her dual identity as a sentient tapeworm inhabiting a human body. She becomes a bridge between humans and the "chimeras," the sentient parasites who have taken over their hosts but retained their intelligence. Sal is caught between her loyalty to humanity, her kinship with the chimeras, and her quest to stop SymboGen, the corporation that unleashed this disaster.

The novel explores themes of identity, survival, and morality as Sal and her allies fight to protect both human and chimera lives. SymboGen’s manipulations and attempts to contain the crisis lead to devastating consequences, forcing Sal to make impossible decisions. The stakes rise as Sal works to find a path forward that acknowledges the complex coexistence of humans and chimeras.

With a mix of action, horror, and thought-provoking ethical dilemmas, Chimera ties up the trilogy's major storylines while leaving readers to consider the implications of humanity’s pursuit of scientific advancement.