Jackie Todd is 27 years old, with sly eyes, a laugh that seems to come from deep in her belly, and thick, dirty-blond hair that she dyes a fiery copper hue. She also has a small computer inside of her chest. It’s constantly collecting information; it’s a diary of dates, times, and events. In a sense, Jackie’s whole life is archived in a code that she can’t interpret. She jokes that she’s part cyborg, but it’s not entirely a gag: a $50,000 machine is keeping her alive. “This device will do everything it can to prevent my heart from stopping,” she says.

The first time Jackie’s implanted defibrillator shocked her heart back into a regular rhythm, she had been sprinting across her high-school campus. It was late fall, about 45 degrees—the air chills early in Chicago. “It felt like daggers poking into my lungs,” she told me. Her sisters had warned her that she needed to hurry up to make a screening of the second Harry Potter film. Jackie was 14. She’d had an implanted device for two years.

“When it fired, it was as if someone had shoved metal doors violently into my back,” she says. “It knocked the wind out of me.” Her heart rate had skyrocketed to 275 beats per minute, and the initial shock didn’t jar it back to normal. That took another jolt. Her vision went white, her knees buckled, and she collapsed at the top of a stairwell, unconscious.

* * *

It’s a cliché to say that Jackie doesn’t seem sick—but she doesn’t, at least not at first glance. She has a chronic illness, but she’s not frail. An aspiring filmmaker, she graduated from the University of Chicago and has managed to find steady work filming and editing videos for her alma mater. She lives independently. She’s able to exercise (Although, she adds, “I constantly check my pulse. It’s a nervous tic.”) She’s currently working on a documentary about people with implanted defibrillators, entitled Diary of a Cyborg. All of that belies how ill she really is. Something is wrong with Jackie, definitely. But exactly what—and why—has eluded her teams of doctors for nearly two decades.

In addition to her life-threatening cardiac condition—the specifics of which remain distressingly cloudy—Jackie has a diagnosable connective tissue disorder, Ehlers-Danlos syndrome. It may be co-morbid. It’s likely progressive. There’s something screwy about her collagen production, which makes her skin and joint sockets too elastic. Flexibility was an asset when she was working on contortionist routines for the campus circus—but it can also be dangerous. For instance, her joints are precariously unstable. “There are days when my shoulders will just slip out of their sockets,” she says. She regularly writhes with muscle cramps or spasms and relies on pain medications to help her sleep.

The relationship between her illnesses—if there is one—is slippery. The similarity is that her conditions fall within the category of invisible illnesses, the symptoms of which may not be apparent to an onlooker. Ehlers-Danlos can often be diagnosed by a physical exam and genetic testing, but the symptoms—such as fragile, stretchy skin—don’t necessarily register as visual cues of illness if someone isn’t looking for them.

Patients presenting with complaints of joint pain or rashes may initially be turned away by primary care physicians before ending up in a rheumatologist’s office. “Common things are common,” notes Amit Saxena, a rheumatologist at New York University’s Langone Medical Center who specializes in autoimmune diseases. It’s logical to assume that most patients with joint pain probably don’t have lupus, or multiple sclerosis—but some of them might, especially if they have a genetic predisposition coupled with a triggering event (though those are hard to pin down, too). “It can be a challenge to put it together. It’s not always an exact science,” Saxena admits.

In the absence of definitive medical tests for some invisible illnesses, such as fibromyalgia, patients fight to be taken at their word—to have doctors, friends, and family believe that they’re reliable narrators of their own experiences. Many practitioners diagnose fibromyalgia, for instance, based in part on the presence of 18 tender points throughout the body (such as the elbows and inner sides of the knees), which are acutely painful when touched. Of course, that experience is more subjective than quantitative. The burden of proof often falls squarely on the ill person.

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.

Mike was first hospitalized in 1983, after complaining of sweating and rapid chest palpitations. These happened, Janice wrote, “suddenly and with no pattern.” He was hospitalized again five years later. His brother, too, had experienced similar symptoms over the course of 10 years: repeated bouts of palpitations and sweating. Two cardiologists diagnosed Mike’s problems as stress-related. But stress doesn’t kill people on top of stationary bikes.

“I have three children (ages 10, 7, and 5) who I am most concerned about,” Janice wrote. She enclosed two EKGs, a 24-hour Holter report (the data from a portable monitor that records the heart’s electrical activity around the clock), two echocardiogram results, and three pages of notes on treadmill-induced stress tests. She wondered whether her daughters—Lauren, Allison, and Jackie—should receive EKGs, just to evaluate their baseline heart function. Those tests kicked off a 20-year quest for a diagnosis and treatment plan that could save the girls’ lives.

* * *

After Mike died, Janice—a nurse—became obsessed with genealogy. She started gathering data from the extended family, and piecing together a blurry puzzle. She was combing histories for patterns, or red flags. “Despite what the doctors told my mother, ‘Don’t worry, your daughters will be fine,’ she kind of always had a feeling that this wouldn’t be a one-time thing with my father,” Jackie recalls. Janice didn’t write off symptoms like dizziness the way some parents might—maybe the child was dehydrated, or spent too much time in the sun. Instead, she enlisted a team of pediatric cardiologists.

The girls started to display symptoms around the onset of puberty. Jackie’s older sister, Allison, was the first member of the family to receive a tentative diagnosis. It was 2008, and she was 13. It began when she was complaining of “not feeling right in gym class,” Janice wrote. “Allison told me that it felt like her head was going to come off.” The doctors at Hope Children’s Hospital in Oak Lawn, Illinois suggested that she had long QT syndrome (LQTS), or at least something that resembled it. Could that have been what killed her father?

The Mayo Clinic—where Jackie and her sisters went for a three-day battery of diagnostic tests after Allison’s incident—describes long QT syndrome as a heart rhythm disorder that causes “chaotic” beats. A heartbeat is measured in five waves, which correspond to letters—P, Q, R, S, and T. During an electrocardiogram, doctors can evaluate the length of time it takes the muscle to contract and refill with blood. That’s the Q-T interval. Essentially, people with LQTS have hearts that take too long to recharge between beats. Erratic, irregular beats can trigger fainting spells or seizures and lead to sudden death. The disorder is often caused by a gene mutation. So far, 12 genes have been identified as associated with LQTS. In other cases, medications such as anti-histamines, diabetes drugs, and antidepressants can throw off the heart rhythm.

The Todd sisters have donated blood to research studies and have also had sequencing performed for the kinds of LQTS that are currently associated with specific genes. Those tests came back negative. The girls had EKGs and echocardiograms, and sent their test results to doctors on the East Coast and in Italy. The symptoms didn’t neatly align with any existing diagnosis, but Allison and Jackie continued to display troubling signs. Allison often has slow heartbeats. She’s exhausted. She has very low blood pressure. Jackie’s heart races, even when she’s sitting still. The doctors’ best guess is that they have a genetic condition that closely resembles LQTS, and could be fatal.

The first line of treatment was a cocktail of beta-blockers. But the girls struggled with lethargy, chronic exhaustion, and, sometimes, depression. Ultimately, the Todds concluded that their quality of life would be improved if the sisters traded the medication for implanted devices. After a five-hour surgery, in which a defibrillator was placed in a little flap—“kind of like a breast implant,” Jackie says—Jackie and Allison had built-in regulators. The devices have kept the girls alive, but Jackie still doesn’t have an answer to a question that feels fundamental: What’s going on in there?

* * *

Coupled with pain, that sense of estrangement takes a toll. “Some days, it just feels like full-on betrayal—like, suddenly, my body is falling apart,” Jackie says. “I wonder, how much longer can I do this?”

“Part of me would love to have a diagnosis,” she continues. “But what would it really change? We have data that shows that, sometimes for no reason whatsoever, my heart shoots up to a ridiculous rate.”

And since the mysterious heart problem is only suspected to be genetic, there’s no way to test for it or predict its severity—doctors can’t tell who might be a carrier, or who might present symptoms later on. That terrifies Jackie. What if she has kids? Will she be able to keep up with them? And that perpetuates the cycle of vigilance that her own mom displayed—an obsessive attention to passing complaints that may have saved her daughters’ lives. But it also fosters a sense of doom and paranoia. “I have a nephew who I’m just over the moon about,” Jackie says. “We don’t know if his parents are carriers for the gene. We don’t know anything, really. We just have to watch him.”