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.

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