In 2009, Susannah Cahalan was a healthy 24-year-old reporter for the New York Post, when she began to experience numbness, paranoia, sensitivity to light and erratic behavior. Grasping for an answer, Cahalan asked herself as it was happening, "Am I just bad at my job — is that why? Is the pressure of it getting to me? Is it a new relationship?"

But Cahalan only got worse — she began to experience seizures, hallucinations, increasingly psychotic behavior and even catatonia. Her symptoms frightened family members and baffled a series of doctors.

After a monthlong hospital stay and $1 million worth of blood tests and brain scans that proved inconclusive, Cahalan was seen by Dr. Souhel Najjar, who asked her to draw a clock on a piece of paper. "I drew a circle, and I drew the numbers 1 to 12 all on the right-hand side of the clock, so the left-hand side was blank, completely blank," she tells Fresh Air's Dave Davies, "which showed him that I was experiencing left-side spatial neglect and, likely, the right side of my brain responsible for the left field of vision was inflamed."

As Najjar put it to her parents, "her brain was on fire." This discovery led to her eventual diagnosis and treatment for anti-NMDA receptor encephalitis, a rare autoimmune disease that can attack the brain. Cahalan says that doctors think the illness may account for cases of "demonic possession" throughout history.

Cahalan's new memoir is called Brain on Fire: My Month of Madness.

On the moment when Cahalan lost her sanity

"I don't remember anything from this experience. This was all told to me after the fact. [My boyfriend] Stephen heard guttural sounds coming from me. He thought maybe I was just angry because I hadn't slept for days, and he knew that it was really frustrating. And so he thought, 'Maybe she's just venting her frustration.' But the grunts were very unnatural sounding, so he turned and looked at me. And he saw that my eyes were wide open but completely unseeing, and at that point he tried to shake me and say, 'Are you OK, Sue? What's going on?' And at that point, my arms whipped out, and I had a grand mal seizure, and I was convulsing. And I bit my tongue so that blood and kind of a combination of blood and foam was coming out of my mouth. And he had the presence of mind — and I think this is incredible — to know that this was a seizure because I had never had a seizure before. And so he turned me on my side and he called 911. And [that's, for me] ... the difference between sanity and insanity ... that moment where kind of my memory goes dark."

On some of the symptoms she exhibited at the hospital

"I slurred my words. I drooled. I didn't have proper control over my swallowing ... I kept my arms out in unnatural poses. At one point, I was like the Bride of Frankenstein — I kept my arms out rigidly. I was slow. I could hardly walk, and when I did, I needed to be supported ... I started [acting] very psychotic. I believed that I could age people with my mind. If I looked at them, wrinkles would form, and if I looked away, they would suddenly, magically get younger. And I believed that my father had murdered my stepmother. I believed all these incredibly paranoid — a huge, extreme example of persecution complex. And then as the days went on, I stopped being as psychotic, and I started entering into a catatonic stage, which was characterized by just complete lack of emotion, inability to relate, or to read, or hardly to be able to speak."

On being supported by her parents and boyfriend

"Without them, I wouldn't be here right now, especially with my mom. She was a bulldog. I mean she would not take 'no' for an answer, especially in the beginning when they were saying it was alcohol withdrawal and partying. She refused to see that as an answer, and so she did her own research. She asked questions. At home, after a day at the hospital, she'd make a list of all the different terminology they used, and she'd look it up and, you know, not everyone is capable of doing that. ... I was so lucky to have someone there for me that could do that. ... If everyone could have someone like that, it would just be a better world."

On the possible connection between her rare immune disease and cases of "demonic possession" throughout history

"When you think about the symptoms — in my case alone, this grandiosity, this violence. In a lot of children, you see hypersexuality. Even my grunts and these guttural sounds that came from me sounded superhuman to someone who might be inclined to think that way. ... When you see videos of people — in fact, when I see videos of myself — demonic possession is not far from your mind. It wasn't far from Stephen's mind when he first saw that seizure. And I've talked to many people who've had this disease, and one woman I spoke to actually asked for a priest because she said, 'The devil is inside of me. I need it out.' A little girl was grunting — they had a monitor in her room — and she was grunting so unnaturally that her parents looked at each other and said, 'Is she, is she possessed?' They actually said that about a little girl. You can see throughout history why people would believe this."

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