CALGARY, Alberta — Aidan’s tics erupted one day after school in early 2021, about a month after the long pandemic lockdown had ended. The 16-year-old convulsed while walking into the house, head snapping and arms swinging, sometimes letting out high-pitched whistles and whoops.Aidan’s parents looked up from the living room couch with alarm. They had been worried about the teenager’s ratcheting anxiety — related to Covid, gender dysphoria, college applications, even hanging out with friends. But they were not prepared for this dramatic display.“We watched this happen in front of our eyes,” Aidan’s mother, Rhonda, recently recalled. “It looked like Aidan was going crazy.”They rushed Aidan to the emergency room, but doctors found nothing wrong. After calling a neurologist, the family learned that more than a dozen adolescents in Calgary had recently come down with similar spasms.Over the next year, doctors across the world treated thousands of young people for sudden, explosive tics. Many of the patients had watched popular TikTok videos of teenagers claiming to have Tourette’s syndrome. A spate of alarming headlines about “TikTok tics” followed.But similar outbreaks have happened for centuries. Mysterious symptoms can spread rapidly in a close-knit community, especially one that has endured a shared stress. The TikTok tics are one of the largest modern examples of this phenomenon. They arrived at a unique moment in history, when a once-in-a-century pandemic spurred pervasive anxiety and isolation, and social media was at times the only way to connect and commiserate.Now, experts are trying to tease apart the many possible factors — internal and external — that made these teenagers so sensitive to what they watched online.Four out of five of the adolescents were diagnosed with a psychiatric disorder, and one-third reported past traumatic experiences, according to a study from the University of Calgary that analyzed nearly 300 cases from eight countries. In new research that has not yet been published, the Canadian team has also found a link to gender: The adolescents were overwhelmingly girls, or were transgender or nonbinary — though no one knows why.Perhaps as striking as the wave of TikTok tics is how quickly it has receded. As teenagers have resumed their prepandemic social lives, new cases of the tics have petered out. And doctors said that most of their tic patients had now recovered, illustrating the expansive potential for adolescent resilience.“Adolescence is a period of rapid social and emotional development,” said Dr. Tamara Pringsheim, a neurologist who co-led the studies in Calgary. “They are like sponges, grabbing onto new skills to cope.”Curious ClustersRhonda received a hug from Madelyn, Aidan’s cousin, after describing Aidan’s bout with the tics during a family dinner in December.Historians looking back thousands of years have come across stories of patients — most often women — with tremors, seizures, paralysis and even blindness that could not be explained. The ancient Greeks called it “hysteria” and blamed a wandering uterus. Sigmund Freud deemed the condition “conversion” and theorized that it was caused by suppressed traumatic experiences.In more recent decades, scientists have gained a greater understanding of how anxiety, trauma and social stress can spur the brain to produce very real physical symptoms, even if body scans or blood tests show no trace of them. When these illnesses interfere with day-to-day life, they are now called “functional disorders.”“We all recognize that the mind can make the body do things,” said Dr. Isobel Hayman, a child and adolescent psychiatrist at the UCL Great Ormond Street Institute of Child Health in London, who published the first report on the pandemic tics. Most people, after all, have experienced fear that makes their heart race or anxiety that ties their stomach in knots.“But when the symptoms are quite bizarre and quite intense — like a seizure, or not being able to walk, or ticlike movements — we think, ‘How on earth can the brain generate symptoms like this?’” Dr. Hayman said. “It just can.”These sudden symptoms can also spread in clusters, reflecting the shared pressures on a group. In the Middle Ages, a period when many Europeans feared being possessed by the devil, nuns living in a French convent began meowing like cats. In the 2000s, hundreds of children of asylum seekers in Sweden became mute and bedridden for months to years.But ask any neurologist about the TikTok tics and they will bring up Le Roy, a small town in western New York. In 2011, a cheerleader at the local high school erupted in a fit of spasms. A few weeks later, her best friend began snapping her head. The tics spread quickly through the social hierarchy at the school, affecting 18 girls, one boy and one adult woman.Tips for Parents to Help Their Struggling TeensCard 1 of 6Are you concerned for your teen?