'Symbiont' (1954) by Mira Grant

'Symbiont' is the second book in Mira Grant’s 'Parasitology' series, following 'Parasite'. It continues the story of a world transformed by the SymboGen "Intestinal Bodyguard," a genetically engineered tapeworm designed to protect human health. However, the seemingly miraculous parasite has begun causing catastrophic consequences.

The story picks up as more people infected with the tapeworms succumb to "sleeping sickness," a condition in which the parasites take full control of their hosts’ bodies, turning them into violent, zombie-like creatures. Sal Mitchell, the protagonist, discovers that she is not entirely human but a sentient tapeworm in a human body -a revelation that challenges her identity and her place in the unfolding chaos.

As Sal and her allies work to survive and find a solution to the escalating crisis, they face threats from the infected, government forces, and SymboGen, the corporation behind the parasites. Sal must navigate dangerous alliances, uncover hidden agendas, and make heartbreaking choices to save herself and the people she cares about.

The novel delves deeper into the ethical and existential questions raised by bioengineering, personal identity, and the unintended consequences of technological advancements. Symbiont delivers a mix of suspense, horror, and scientific intrigue, setting the stage for the series' explosive conclusion in Chimera.