Strep throat, a bacterial infection of the throat and tonsils, is a common childhood illness that affects millions each year. Most cases of strep clear up without any lasting aftereffect.

But in a small number of cases, the infection triggers an autoimmune response in the body that in turn causes a number of serious ills and symptoms, researchers say. These include obsessive-compulsive disorder (OCD), tics, anxiety, irritability, hyperactivity, anorexia, and urinary problems.

“Parents have used the word ‘possessed,’” says Susan Swedo, chief of pediatrics and developmental neuroscience at the National Institute of Mental Health who first associated strep throat with these changes. “Their sweet, wonderful child turns into a monster seemingly overnight.”

Scientists studying this connection believe this response occurs in people predisposed to what a condition called PANDAS (Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcus).

Any child who suddenly develops OCD-like symptoms should be tested for strep, recommends the International OCD Foundation, based in Boston. But the American Academy of Pediatrics recommends testing for strep only when strep symptoms are evident – such as throat pain, difficulty swallowing, red and swollen tonsils.

It is estimated that approximately 1 percent of children in the U.S. have OCD.