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.'"