Illustration: Mauricio Alejo

In April 2009, Susannah Cahalan, a 24-year-old reporter for the New York Post, woke up strapped to a bed in a hospital room. She had no clear memory of the previous few weeks, though her medical records showed that she'd been psychotic and violent before lapsing into a profound catatonia. Her doctors had ordered a battery of blood tests and brain scans, but they revealed nothing. It took the brilliant neurologist Souhel Najjar, MD, to find the cause: Cahalan had a rare disease that caused her immune system to attack her brain. In her new book, Brain on Fire, Cahalan chronicles her terrifying ordeal and the desperate search for a cure. We asked her to walk us through her journey.

Q: Did you know you were sick before your hospital stay?

A: I knew something was wrong; I was constantly tired and I'd developed numbness on my left side. I'd also become paranoid that my boyfriend was cheating on me. I thought I was having a nervous breakdown. One psychiatrist told me I was bipolar. Then one day I walked through Times Square and the lights were painfully bright. I was experiencing photophobia, which preceded a massive seizure.

Q: When Dr. Najjar came in, he asked you to draw a clock face on a piece of paper. How did that help him diagnose your condition?

A: When I drew the clock, I squeezed all the numbers into the right half of the circle. My brain wasn't "seeing" the left half, which was a sign that the right hemisphere of my brain was inflamed. My doctors already suspected that I had an autoimmune disease, but the test enabled Dr. Najjar to finally connect all the symptoms—paranoia, psychosis, increased heart rate, and numbness on my left side—into a single diagnosis.

Next: What the test revealed about her brain