It’s the first time you have felt hope since the nightmare began. You want to spring out of your chair and hug the man for his determination to rescue you and your boy from this frightening wilderness.

 I know this case intimately because that boy is my nephew.

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ONE OF THE SCARIEST PARTS about PANDAS is also what makes the diagnosis so controversial. Its trigger is the common strep infection, which is as reliable a part of the grade school experience as recess.

Dr. Robert Fuhlbrigge, a pediatric rheumatologist at Boston Children’s Hospital and associate professor at Harvard Medical School, puts it this way: “Virtually every child of school age gets strep every year.” If that seems impossible, consider that he’s including the many kids whose reaction to strep is mild enough that it is mistaken for the common cold. And he’s including the estimated 12 percent of schoolchildren who are carriers, meaning they aren’t bothered by streptococcal bacteria, yet can spread it to susceptible kids when they sneeze or cough.

Only those children who produce a positive throat culture are actually treated for strep. And the penicillin they get isn’t even for the fever and sore throat, which will usually go away in a few days. The antibiotic, Fuhlbrigge says, is primarily to guard against the very small risk that the strep infection will develop into rheumatic fever, and then rheumatic heart disease.

Though relatively uncommon in this country, rheumatic heart disease is rampant in the developing world. What causes it? Basically, the immune system gets confused. In trying to fight the strep bacteria, it mistakes a protein on the surface of heart valves for a similar looking protein on the surface of a strain of strep. Then it begins attacking the heart valves.

The idea behind PANDAS is that it works like rheumatic fever, except that rather than attacking the heart, the immune system goes after the brain. In the early 1990s, researchers at the National Institutes of Mental Health (NIMH) began studying whether infections could be behind acute-onset OCD in children. They focused on strep because it was the same bacteria behind an established neurological disorder called Syndenham chorea, where a portion of those who develop rheumatic fever begin to make involuntary, jerky movements. In 1998, a NIMH team led by Dr. Susan Swedo published clinical descriptions of 50 children whose sudden OCD and tic symptoms had been preceded by a strep infection.

Many researchers, including rheumatologists like Fuhlbrigge, saw promise in the hypothesis, because it followed the logic of rheumatic fever. Others, particularly neurologists, were dubious. Strep is so common that you’ll find it any classroom. Why not just argue that recess causes OCD?

The sniping in the medical field persisted for more than a decade, with subsequent studies unable to produce clear answers. Even Fuhlbrigge began to wonder whether Swedo had it right — the scientific evidence that strep causes OCD-like symptoms just wasn’t there. Childhood OCD, with its gradual onset, is fairly common — an estimated 1 percent to 3 percent of children have it. Fuhlbrigge and others came to suspect that PANDAS cases may simply be OCD children on the extreme end of the spectrum who respond particularly poorly to certain stressors, such as the stress caused by an infection.