“She came across as genuine, loving, and funny,” says Angelacos. “No one questioned her authenticity, including me—and I usually have a pretty active BS radar.”

Marchand’s posts gradually became more extreme, the group’s members say. She wrote that her son was being bullied over her diagnosis, and that her dog had been shot. Then, in December, according to Angelacos, Marchand announced that she was out of treatment options. Her cancer had spread to all of her major organs. She didn’t have much time left to live. Soon, she stopped posting.

Angelacos assumed Marchand had become another tragic cancer statistic. But when Angelacos reached out to Marchand’s family to check in, she was shocked to learn that Marchand was alive—and apparently healthy.

Around the time Marchand stopped posting in the Facebook group, she was arrested in Colorado for faking terminal cancer on the crowdfunding platform GoFundMe and accepting donations through multiple accounts. It seemed she had faked her illness to the Facebook group, too. At trial, she pleaded guilty and was sentenced to community service. “The entire group was devastated, angry, and in a state of disbelief,” Angelacos says. “Everyone felt they had come to know her so well. There was a huge sense of betrayal.” (Marchand and her lawyer did not respond to requests for comment.)

This was not the first time many of the group’s members had felt this way. As harrowing as the experience can be for those involved, people in online cancer support groups are routinely outed as healthy. It’s difficult to speculate exactly how common this phenomenon is: There have been no large-scale scientific investigations into the internet’s cancer fakers, and the evidence is limited to only those who have actually been suspected or caught. But among the internet’s cancer communities, it’s an often acknowledged problem, albeit still a shocking one. Among 10 people from three groups I spoke with recently, every person recalled someone being outed for faking in their communities at least once, if not more.

Read: When people seem to want to be sick

Several recent high-profile cases have highlighted the issue. In 2015, the wellness blogger Belle Gibson confessed that she lied for years about a brain-cancer diagnosis. Last year, the then nanny Candace Ann Streng was sentenced to up to 15 years in prison after pretending to have terminal cancer and scamming $30,000 via a GoFundMe campaign. In February, a New Yorker profile accused the best-selling author Dan Mallory of faking brain tumors to colleagues several years ago. (Mallory claimed after the story was published that he’d feigned brain cancer to disguise struggles with bipolar disorder.)

This condition of faking illness online has a name: “Munchausen by internet,” or MBI. It’s a form of factitious disorder, the mental disorder formerly known as Munchausen syndrome, in which people feign illness or actually make themselves sick for sympathy and attention. According to Marc Feldman, the psychiatrist at the University of Alabama at Tuscaloosa who coined the term MBI back in 2000, people with the condition are often motivated to lie by a need to control the reactions of others, particularly if they feel out of control in their own lives. He believes that the veil of the internet makes MBI much more common among Americans than the 1 percent in hospitals who are estimated to have factitious disorder.