It's a memoir of her struggle with mental illness -- she refers to it as a month of madness.
I'm not sure whether it's even accurate to call it mental illness, since (no spoilers, this is clear from the beginning) they uncover a cause that is autoimmune, but in the most literal sense, it's certainly an illness affecting mental health.
Anyway, this was the scariest thing I've ever read. First, the kind of loss of control and autonomy associated with being in an institution -- not even necessarily a mental institution, even a hospital, but much more so when you can't just choose to leave -- is scary to me. Throw in not being able to control or trust your brain, having a doctor write down false information about you and dismiss your illness as alcoholism, and being ill with something rare enough and little enough known that getting diagnosed is a matter of luck (and luck leaning on privilege at that) and it's a true horror story. Freddy Krueger has nothing on this.
Furthermore, a common problem with true stories is that not everyone is a writer. In Cahalan, the reader is blessed with a storyteller who knows how to tell a story. She draws you in and even when discussing technical details, her work reads like a story, in a place where it could have easily sounded like a chapter you were required to read in a class you hadn't wanted to take.
I was surprised to hear it over so fast, and I will say I wasn't left with a sense of relief at the end. This isn't a horror story where the monster is slain at the end -- the reader knows it could come back and could be lurking in any of us, waiting to attack.
Bonus: narrator Heather Henderson renders the story beautifully. She sounds like your friend sitting on your sofa sharing. Her reading holds emotion that carries the narrative without ever going overboard and detracting from it.