Susannah Cahalan found two red dots on her arm one morning in February 2009. She figured she had bedbugs like many New Yorkers at the time. After several fumigations and increasing anxiety, Cahalan’s mental state unraveled. She was obsessed.

 “Looking back, that seemed to be the beginning of these paranoid thoughts that started entering,” she said.

The then-24-year-old grew increasingly fearful. As the red marks on her arm faded, her paranoia escalated.

Cahalan’s symptoms mirrored bipolar disorder or schizophrenia early on, but later the writer’s erratic behavior gave way to violent seizures. Blood and foam spurt out of her mouth at one point, prompting her family to seek a neurologist.

During a month-long hospital stay, she was strapped to her bed and nurses had to tackle and sedate her after she attempted to escape her room.

Cahalan documented the experience in her memoir, “Brain on Fire: My Month of Madness,” which is out in paperback this week. Looking back, Cahalan said she still struggles to identify with her old self.

“The way I feel about it is that a very close friend went through it,” she said. “It’s not a stranger per se, but it’s not me.”

Cahalan consulted doctors, family and friends to piece together “Brain on Fire,” but later realized they left out some grim details. Medical records and surveillance video documenting her stay revealed the grave nature of her madness.

Because the young New York Post reporter could not quite come to grips with the disease, Cahalan originally needed to dissociate herself from the writing process.

“For the reader, it didn’t work,” she said. “But for me, I had to write initially in third person. That’s how I felt about it.”

Cahalan was misdiagnosed several times before she found out she had a rare illness called anti-NMDA receptor encephalitis. One doctor quickly pronounced Cahalan was simply experiencing alcohol withdrawal. Another pointed to schizophrenia. Her symptoms, from psychotic behavior and catatonia, eventually led her to Dr. Souhel Najjar.

The neurologist closely monitored Cahalan and decided the illness was physiological. When Najjar drew the outline of a circle on a notepad for Cahalan, he told her to draw the numbers of a clock. After she drew one through twelve on only the right side of the clock, Najjar knew he had a clue.

 “No one with a psychiatric condition would draw a clock like that,” Cahalan said. “It also is proof that the right side of my brain was impaired in some way.”

Najjar said Cahalan’s brain was “on fire.” To quench the flames, Najjar quickly put her on a series of steroids and medications to alleviate the swelling in her brain. She recovered within a year of the diagnosis.

“Brain on Fire” weaves a medical mystery with a compelling memoir, a scientific story palpable for most readers. Cahalan’s memory might not fill in all the blanks of her lost month for a truly personal recollection, but the factual tale catches fire in its poignant conclusion.

The book sheds light on the harrowing consequences of misdiagnosis. Alluding to the fate she narrowly avoided, Cahalan’s story appeals to the stark difference in treatment of a mental illness.

“The care was different,” she said. “It wasn’t overt, but there was a looming threat that they were going to send me to a psychiatric ward.”

What: Susannah Cahalan reading

Where: Magers & Quinn, 3038 Hennepin Ave. S. Minneapolis

When: August 16, 7 p.m.