The following version of this book was used to create this study guide: Beams, Clare. The Illness Lesson. New York: Doubleday, 2020.

The Illness Lesson is set in 1871, where 29-year-old Caroline Hood is living with her father, Samuel, a famous essayist, on a farm in Ashwell, Massachusetts. She is surprised by the sudden appearance of a strange bird that used to frequent the farm when her mother, Anna, was still alive, 25 years earlier. Anna had called this type of bird the Trilling Heart because it reminded her of a disembodied heart. Anna died as the result of an epileptic seizure. Caroline and Samuel are joined by David Moore, a young man who has come to Ashwell to be Samuel's protege. Samuel and David tell Caroline they have conceived of an idea to open a school for girls on the farm, where the students would receive an education commensurate with that offered to boys, quite a revolutionary concept. They wish to call it the Trilling Heart School and ask Caroline to teach literature.

Plans for the school develop quickly and students begin enrolling. One of these students is Eliza Pearson Bell, the daughter of Miles Pearson, a man who lived at Birch Hill during Samuel's first venture, a collective farm. There were rumors that Miles had an affair with Caroline's mother Anna, and he even wrote a romance novel called The Darkening Glass inspired by his feelings for her. Caroline, meanwhile, develops romantic feelings for David.

One day, while out on a walk, Eliza faints. Caroline grows concerned that she might have epilepsy like Anna. She is surprised to learn David is married, information he has obviously been intentionally keeping from her. His wife, Sophia, joins the others on the farm to teach art. The students are gathered in the barn where they have classes. As Sophia is being introduced, a piece of the ceiling falls in. It had been weighed down by a Trilling Heart nest. Caroline grows increasingly frightened of the birds and their omnipresence. That night, Caroline hears the students sneaking out of the farmhouse. She follows them quietly and observes as Eliza lights a stick on fire with a lantern, then presses it against her skin. She encourages the other girls to do the same, but before they can, Caroline makes a sound in the woods that causes them to disperse.

Next, Eliza develops a strange red spot in her eye. Caroline and Samuel agree she should be sent home, but she faints in the carriage and David brings her back to the farm. A few days later, another student develops a rash on her arms. Soon after, all of the other girls are affected by strange symptoms of illness. Caroline wishes to send them all home, but David reminds her that her father's professional reputation is at stake. She reluctantly agrees to Samuel's suggestion that they invite his friend Dr. Hawkins, whom Caroline has always despised, to the farm to consult.

Dr. Hawkins arrives from Boston and meets with the girls. Caroline, meanwhile, begins developing strange symptoms of her own, feeling a tingling in her fingers and even fainting while on an errand in town. Hawkins diagnoses the girls with hysteria and recommends a treatment of “hysterical paroxysm” (191). It is gradually revealed that this treatment involves sexually violating the girls. Sophia is disgusted and demands the treatment be stopped. When no one listens to her, she returns home to Ohio. Hawkins insists on performing the same treatment on Caroline after seeing her terrified reaction to a Trilling Heart bird flying near her. Afterwards, Caroline insists that the treatments stop. She goes to David's room to ask him to help her talk to her father. He agrees and they make love. However, when they actually speak to Samuel, David quickly backs down and takes Samuel's side. Later, Samuel tells Caroline that her mother did not die of epilepsy; she fell from the loft in the barn. Caroline determines that her mother must have jumped.

Furious that no one is supporting her and that she has been lied to all her life about her mother, Caroline writes a letter to the newspaper announcing the closing of the Trilling Heart School. She then packs her things and prepares to move to Boston. Just before she leaves, she discovers a giant Trilling Heart nest in the forest that is filled with fragments of objects belonging to the girls—scraps of paper and fabric, and even a human tooth. She considers burning the nest but decides to leave it there instead, a monument to the girls' suffering. After moving to Boston and getting a job teaching at a new school, Caroline, runs into Sophia one day and learns that David is running for a position in the Ohio legislature. As she settles into her new life, Caroline begins to feel some measure of closure about her mother's death and the sexual abuse she witnessed and endured.