By Stephanie Stahl

PHILADELPHIA (CBS) – It’s a mysterious, newly discovered disease that strikes mainly young women, and it’s often misdiagnosed. Doctors who discovered it, here in Philadelphia, say it’s like your brain is on fire. 3 On Your Side Health Reporter Stephanie Stahl says it starts with personality changes.

Young women dazed, restrained in hospital beds, acting possessed and then becoming catatonic. They’d been so normal, when suddenly their lives went haywire.

“One minute I’d be sobbing, crying hysterically, and the next minute I’d be laughing, said Susannah Cahalan, of New Jersey.

“I was very paranoid and manic. There was something wrong. I thought trucks were following me,” said Emily Gavigan, of Pennsylvania.

And it got worse for Emily Gavigan, who was a sophomore at the University of Scranton. Hospitalized, and out of it, she couldn’t control her arm movements. Then there were seizures, and she needed a ventilator. Her parents were watching their only child slip away.

“It was life and death for weeks,” said Grace Gavigan, Emily’s mom.

“We were losing her. This is something that I couldn’t control,” said Bill Gavigan, Emily’s dad.

Doctors also couldn’t figure out what was wrong with Susannah.

“I had bizarre abnormal movements, would leave my arms out extended, you know, in front of me. I was a relatively normal person, then the next minute I’m hallucinating and insisting that my father had kidnapped me,” said Susannah.

Turns out, Susannah and Emily weren’t mentally ill. They both had an auto immune disease called Anti-NMDA Receptor Encephalitis, when antibodies attack the brain, causing swelling.

Susannah says this is how doctors explained it to her parents, “He told them her brain is on fire. He used those words: ‘Her brain is on fire.’”

A drawing of a clock is how doctors figured out what was going on. Susannah put all the numbers on one side, a clear indication there was a neurological — not a psychological — issue.

“Nobody knew about the condition,” said Dr. Josep Dalmau, a neurologist with the University of Pennsylvania. He first discovered the disease there in 2007.

The rare brain inflammation strikes mainly young women. No one knows why.

“A frequent mistake is to think that the patient has a primary psychiatric disorder. There are many patients that I still think that they are misdiagnosed,” said Dr. Dalmau.

Susannah and Emily were both eventually correctly diagnosed at Penn. Researchers there developed both a spinal fluid test to diagnose the disease, and an immunotherapy to treat it. That can take weeks even months.

Susannah, who’s completely recovered, is back and reunited with her dog in New Jersey. She just published a book about her journey called, Brain On Fire: My Month of Madness.

“You know, thousands of people have now been diagnosed, so it’s a rare condition, but it’s not exceedingly rare. It’s not as rare as initially thought,” said Susannah.

Emily and her parents now want to warn others about the mysterious disease.

“There could be people in comas right now or people stuck in psych wards that have this disease and aren’t being treated properly,” said Emily.

At this point, there is no cure. Emily and Susannah could relapse, which would require a new round of treatments.

Penn recently released a new study about the disease.

For more on Penn Medicine, visit: www.pennmedicine.org

For more on Susannah Cahalan, visit: www.susannahcahalan.com