'Parasite' (1954) by Mira Grant

'Parasite' is the first book in the Parasitology Series. It is set in a near-future world where a company called SymboGen has developed a parasitic organism, the "Intestinal Bodyguard," to treat various health issues. The parasite, which lives in the gut, helps prevent diseases and improves overall health by enhancing the immune system. However, this seemingly beneficial organism begins to have unintended, dangerous side effects, including controlling its host’s body.

The story follows Sal, a woman who has survived a near-fatal accident and had a parasite implanted in her to save her life. After her recovery, she has no memory of her life before the implantation and works at SymboGen. However, she begins to experience strange and troubling symptoms. As the parasitic influence becomes stronger, Sal finds herself in a battle for control over her body and life, unraveling a conspiracy involving the parasite, the company, and dangerous experiments.

The novel explores themes of bioengineering, corporate control, and the complex relationship between humans and the technologies they create. It's a tense, thought-provoking thriller that combines elements of science fiction, horror, and medical ethics.