Reporter:
12-year-old Parker's brain is under attack. Mysterious outbursts that look like mental illness. His parents desperate for answers, recording these horrifying moments, hoping that someone might know what's wrong and how to stop it. These fits and attacks all suddenly starting after a bout of strep throat.

Dr. Swedo:
We've known for a very long time the viruses can't trigger mental illness.

Reporter:
Prior Lake, Minnesota, middle-class middle-America. Parker Barnes is the oldest child in a house full of activity.

Parents:
4 kids, 2 dogs, we have a small business and we both work besides that. We have a hectic life.

Reporter:
Parker’s parents, Natalie and Brian, say their son is the kind of kid every parent dreams of having.

Parents:
Parker is affable, relaxed. He's got a dry sense of humor. He's very positive.

Parker:
My nickname that my mom and dad call me is Lee Doug.

Reporter:
But overnight they say he morphed into something else. A year and a half ago when he was 10, Parker got strep throat. Within days, strange symptoms appeared.

Parents:
He had a small throat tic. It just seemed like he never quit clearing his throat. It was minor.

Reporter:
Parker took antibiotics and everything cleared up but over the next 3 months, 3 rounds of strep. Each time, mysterious behaviors appeared and got worse. When he became socially anxious and shut himself off what was your reaction?

Parents:
This is just strange.

Reporter:
That paralyzing anxiety led to obsessive compulsive behavior. Their normally low key boy suddenly tortured by inexplicable rage. The first urgent cry for help came when Parker sent his dad a text message.

Dad:
“Dad I need you to come home right now.” And I ran through the door, he's bawling uncontrollably and I'm like what is going on?  He’s like, “I don't know. I don't know. I can't stop crying. I am horribly sad and I don't know why.

Mom:
I said, “You know, he's been kind of getting like this when he has strep.”

Reporter:
Their doctors recommend seeing a psychologist. Little did anyone know the real crisis was still ahead.  You were about to be blind sided?

Parents:
Oh yeah, blindsided like you never imagined.

Reporter:
One morning Parker and his younger brother Stetson were getting ready before school. I said, “Go brush your teeth. The bus is going to be here in seven minutes. Let's get you ready and out the door.” And he starts screaming. I said, “Oh my god Parker what are you doing?” He’s got a knife.

Brother:
I was the first one to see it. He had a knife. He's about literally to stab himself. He was saying, “I’m useless. I’m useless.”

Mom:
I just grabbed the knife out and I'm just hugging him like, “Honey what's up, what is up with you? This doesn't make sense to me.” He's said, “I just didn't want me to hurt anybody with the strep anymore and he said that and I still didn’t put this together. I just kept saying, “It’s okay, it’s just strep throat, it’s no big deal.”

Reporter:
When you were on your way to school and you picked up a knife, do you remember that?

Parker:
Yeah, I kind of don't remember and I don't really like to think about it.

Reporter:
Panicked, Natalie and Brian called 911. Just hours later, they had to do the unthinkable, admit their 10-year-old son to a psychiatric hospital.

Parents:
We dropped him off and we don't get to stay. We had to leave.

Dr. Swedo:
I just study children and try to understand what's wrong with them and how to help them.

Reporter:
Dr. Sue Swedo works at the National Institute of Mental Health. She believes common childhood infections, like strep throat, can trigger sudden and drastic changes in some children like Parker.

Dr. Swedo:
The wrong strep, in the wrong kid, results in the wrong kind of immune response that impacts the brain and gives rise to behavioral symptoms.

Reporter:
400 miles away just outside Chicago, another family in crisis - Alexia Baire got strep throat for the first time when she was 4 years old. But just days after treatment, her mom, Vanessa, says she changed from a well-behaved, happy child to this.

Child:
Shut up, shut up, shut up!

Reporter:
How long until you saw a change?

Mom:
It was less than 2 days later. It was defiance and some OCD. She just all of a sudden seemed angry.

Reporter:
Alexia was even kicked out of daycare after trashing her classroom. Even she knows something is wrong.

Mom:
She would cry and say “Mommy why can't I be good. I just want to be good.” And, that broke my heart.

Reporter:When you're in the middle of a flare-up, can you sense that something is happening?

Child:
No, not really. I just go and try to hurt people.

Reporter:
The family came to a breaking point when Alexia attacked her own mother.

Mom:
[Driving car] Right up here, I pulled over and I turned to her and as I picked my head up, she was stabbing me in the eye with my mascara wand. All I can see is black and I wasn't sure if she blinded me in the eye or is it the mascara. But it was violent enough and it was intentional.

Reporter:
You had to put your 4-year-old in a psych ward?

Mom:
The hardest 9 days of my life.

Reporter:
Vanessa says doctors sent her young daughter home with a number of psychiatric drugs used to treat schizophrenia and bipolar disorder. Did the drugs work?

Mom:
No, they made her like a shell of her person and then she was still having these bouts of rage and aggression.

Reporter:
It took months before a doctor finally connected Alexia's behavior to strep.

Mom:
And she said every single one of the symptoms that you're describing is a symptom of PANDAS.

Reporter:
If you haven't heard of PANDAS you're not alone. Dr. Susan Swedo first identified it more than two decades ago. It stands for Pediatric Autoimmune Neuropsychiatric Disorders Associated with Strep infections.

Dr. Swedo:
It probably affects somewhere between 1 and 200 and 1 and 500 children every year.

Reporter:
1 in 200 is a huge number of children potentially.

Dr. Swedo:
It is.

Reporter:
Those numbers account for a whole spectrum of cases, ranging from minor to severe, like Alexia and it turns out, Parker too.

Parents:
Good, great, PANDAS.  Whatever it is, we’ll [treat it] and it will be gone.

Reporter:
But treatment wasn’t that easy. Next, inside the cross-country journey to get Parker help.