IMAGINE THAT ONE NIGHT you put your bright, athletic, well-adjusted 8-year-old son to bed, a kid who loves playing baseball and cracking jokes and scarfing down chocolate chip cookies. The next morning, he wakes up as someone entirely different, and in subsequent days turns into someone unrecognizable.

He’s manic, spending hours doing sit-ups or running laps on the driveway — unwilling to sit down even for a minute. He alternates between tears of soul-crushing sadness and tantrums of rage directed at you and your spouse. He’s obsessed with the unhealthiness of food, refusing to eat or drink much of anything. More than anything, though, all the comforting touchstones of his life — home, school, even sleep — have suddenly been transformed into dangers. He seems trapped in a horror movie, his fear unmistakable in the way his pupils have overtaken the irises of both his eyes.

As this bizarre behavior continues, you find yourself staring at your formerly normal, healthy son and you can’t help but wonder, Where did my boy go? You ask yourself: Is this what children of Alzheimer’s patients mean when they talk about looking at a loved one who’s no longer there?

You take your son to your pediatrician, a sympathetic and smart woman who is nonetheless flummoxed. Because some of your son’s symptoms appear to be compulsions, she refers you to a psychologist. Actually, because the need for pediatric mental health treatment dwarfs the supply of mental health professionals, your pediatrician turns to a state referral service called MCPAP, or Massachusetts Child Psychiatry Access Project.

By phone, the consulting psychiatrist instructs your pediatrician to treat your son for obsessive-compulsive disorder, prescribing a Prozac-like antidepressant and recommending that you find a therapist who can guide him through cognitive behavioral therapy to help him “unlearn’’ his new behaviors.

The diagnosis doesn’t sound right to you. Compulsive behavior seems to describe just one strain of his symptoms. And the fact that he literally changed overnight doesn’t compute with what you’ve read about the typical OCD patient, whose behavior changes gradually. Then again, you’re not the expert, and you desperately want your son — your family life — back. So you defer to the professionals.

One day, you find yourself rushing your son to the emergency room. After weeks of unsuccessfully battling with him to eat and drink, he is dehydrated and dangerously underweight. The attending physician comes to talk with you. Peering over his shoulder, you are startled to see the words on your son’s chart: anorexia nervosa. The doctor recommends a book on adolescent anorexia.

“Adolescent?” you ask incredulously. “But my son just celebrated his ninth birthday.”

Yes, the doctor agrees. But you won’t find any books on anorexia for elementary school kids.

When little about your son’s condition improves over the next couple of months, the doctors continue to bump up the dosage of the antidepressant. Because you’ve read enough to know that there are serious questions about the long-term effects of these drugs on the rapidly developing brain of a child, this course of treatment deeply troubles you, especially because it isn’t working. And the cognitive behavioral therapy seems to be equally ineffectual.

As you wait for your son outside the therapist’s office during one appointment, you flip through a copy of Parents magazine and stumble across an article about an obscure condition called Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal infections, or PANDAS. It feels as though it were written about your son. There’s a chart listing seven symptoms of PANDAS, and in your head you can check off every one of them. You ask the therapist about the disorder. Despite the impressive degrees hanging on her wall, she admits she’s never heard of it.

The hypothesis behind PANDAS, you learn, is that it is an autoimmune disorder caused when the antibodies that form to fight a common strep throat infection instead begin to attack the brain. The first step in treatment is to do a throat culture and specific type of blood test. If the results come back positive, the patient’s treatment begins with high doses of antibiotics. Meanwhile, when you do more Googling, you find a scary warning about some PANDAS patients having bad reactions to an anti-anxiety medication called Ativan, which just so happens to be something the consulting psychiatrist also recommended for your son.