Megan Densmore, 31, felt that pressure to convince people that her illness was real when she was diagnosed with fibromyalgia at age 13. (The Centers for Disease Control and Prevention (CDC) reports that more than 5 million Americans have fibromyalgia, and it disproportionately affects women in their childbearing years.) At first, she was in denial. “Even at my sickest, I didn’t look that different,” she says.

And perhaps because she didn’t look incapacitated, despite excruciating muscle aches and cramps, friends and family dismissed her complaints as an attempt to drum up sympathy—or worse, as a fiction. “People told me, ‘This illness is not a real thing, it’s in your head,’ or that this is a bogus diagnosis built on ideas about women being frail or hysterical and trembling and needing smelling salts,” she adds. “They’d accuse me, saying, ‘You just want to be lazy. You just want to whine.’” So Densmore shut down. She didn’t really date, or make friends because she was terrified of being cast as a fraud. “I thought, I don’t even want to broach this topic if you’re going to think that I’m crazy. I’d rather have you think I’m weird or fake than get the real me and think I’m crazy.” (Densmore, an elite competitor in kettlebell sports, is now speaking out as an advocate for people with these sorts of illnesses as the subject of the upcoming documentary Invisible.)

Jackie is familiar with this desire to brush off or minimize symptoms. When she shares what she does understand about her various conditions, it doesn’t always serve her well. Work supervisors have sometimes been less than supportive. One of them told her, “You’re a total liability.” As a freelancer, she didn’t know what recourse she had to respond. She left the job.

These types of illnesses can also create a terrifying schism between the body you think you know, and one that feels foreign. “Until you’re given a reason to be suspicious, people think, ‘Oh, that’s just me,’” she points out. People may assume their experience is universal, or at least within the realm of “normal,” until someone tells them otherwise—especially if their symptoms don’t adhere to an easily-recognizable schema. There’s something that feels so deceitful, so manipulative, about illness that makes itself known but eludes classification.

“All of my tests,” Jackie says, “have led to more questions than answers.”

* * *

Jackie’s father, Michael W. Todd, died on March 12, 1993 at the age of 44. Earlier that month, he’d been complaining of excessive sweatiness, and thought he might have come down with a spring cold or flu. But he felt good on March 11. He worked overtime. He exercised. The next day, he came home from work, chatted with his wife, Janice, and then suddenly passed away while riding on his stationary bike.

His death was assumed to be sudden cardiac arrhythmia. That’s tragic, but not necessarily unusual—the American Heart Association estimates that there are more than 568,000 cardiac arrests in the U.S. each year. (These are different than heart attacks. One is electrical, the other circulatory. In cardiac arrests, the heart malfunctions and stops beating normally, often resulting in swift death. A heart attack is caused by a blockage that prevents blood from reaching the heart.) Cardiac problems—especially coronary artery disease and atherosclerosis—were rampant in the Todd family. Michael’s paternal grandfather died of a heart attack at the age of 52. His uncle died from one at 56. But the thing is, what happened to Mike wasn’t a heart attack. In fact, no one—the coroner included—could figure out exactly what had happened. The coroner reached out to Janice and said he could not determine Mike’s cause of death. And that’s why, in August of 1993, Janice, drafted a letter to Dr. G. Michael Vincent at the Latter Day Saints Hospital in Salt Lake City, Utah.