A few years ago, a man began telling his family members a horrifying tale: There are bugs living inside him.

These bugs, he said, have hard shells that crunch when he squashes them. He can feel them moving around in his body, especially inside his nose and private parts. At first, his family told him—gently—that it wasn't possible, but he just tried harder to convince them.

To collect "samples," the man would dig in his nose with tweezers, pulling out bits of tissue and cartilage until he had bored a hole through his septum; now he whistles when he breathes. After myriad tests and no sign of subcutaneous insects, his doctors seemed to have given up.

He shows the classic signs of what scientists call delusory parasitosis, or Ekbom syndrome, an unwavering but incorrect belief that the patient's body has been infested with something.

For years, entomologists have insisted that these delusions aren't as rare as psychiatrists and the public may think. And now, a study by the Mayo Clinic suggests they're right. The first population-based study of the condition's prevalence suggests that about 27 out of a hundred thousand Americans a year have delusions of an infestation. That would mean around 89,000 people in the U.S. right now are plagued by the condition.

For many sufferers of such delusions, the infestation takes the form of insects or mites, usually tiny and often described as biting or crawling on the skin. Others report feeling worms or leeches or some kind of unknown parasite.

Many of the afflicted turn up, eventually, in an entomologist's office. And as the entomologists tell them, only two kinds of arthropods actually infest humans: lice and a mite that causes scabies. Both are easy to identify and cause characteristic symptoms. Bedbugs or fleas might infest a house, but they don't actually live on or inside the human body; they just feed on us and leave. Likewise, there are mites that live on our skin, especially the face, but they're a normal part of everyone's body, much like the bacteria living in our guts.

Often, there really is an underlying condition causing the itching or crawling sensations that lead people to believe they're infested. Allergies, nutrition, stress, nerve conditions, and reactions to many common drugs can all be root causes. That's one reason not to be dismissive of claims. But what often starts with an itch or other skin or nerve condition becomes a fixation, often on insects.

"There's this anxiety that's prevalent toward arthropods," says Gale Ridge, an entomologist at the Connecticut Agricultural Experiment Station. "So when people believe they've been bitten, they naturally default to that. It's almost instinctive."

Ridge became aware of the man complaining of insects in his nose when a family member emailed her describing his condition. Ridge studies bedbugs and runs the office that handles questions from the public, and she says that in recent years, invisible insects have been taking up much of her time. Last year alone, at least 300 people came to her convinced they're infested with insects, mites, or other creepy-crawlies.

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Each of their stories is unique, but with strong similarities, says Nancy Hinkle, an entomologist at the University of Georgia who has also worked with the condition. One common thread is sometimes called the Ziploc sign—sufferers go to great lengths to collect samples of the "bugs" from their bodies and homes, delivering bags that are often full of nothing but dust, lint, hair, and scabs.

Another similarity is a word they use: desperate. "They call and say, Dr. Hinkle, you have to help me—I'm desperate." Usually by then, they've made the rounds to multiple doctors.

If no physical cause can be found for symptoms, any doctor—even a family physician—can prescribe medications to treat delusional thoughts. But after six months of trying to identify the cause of their anguish, Ridge says, it's harder to sway people toward such treatments.

"A lot of patients won't take them," says dermatologist Mark Davis of the Mayo Clinic, an author on the recent study of delusional infestations. "They say, you're just saying I'm crazy, and I'm not."

In 2012, Davis and his colleagues reported on 147 cases of delusional infestation seen at the Mayo Clinic over seven years. He couldn't think of any patients who he knew had successfully overcome their delusions. Often, he said, they come to Mayo expecting to be diagnosed with an exotic new kind of infection, and they leave disappointed, never to be heard from again.

The internet, meanwhile, has likely served to swell the ranks of the infested. Blogs and websites about various infestations, many of which are devoted to conspiracy theories and biologically impossible explanations, give sufferers a sense of community, but also reinforce delusions and often try to sell them on sham solutions.

Both Ridge and Hinkle say they start every case that comes to them by looking for actual insects or mites. This can mean examining dozens or hundreds of purported samples. If she can't find any evidence of insects, an entomologist must tread lightly. Hinkle says she learned from experience that mentioning mental health professionals is the last thing to do—people just get angry.

Ridge says she's had good outcomes by talking to a sufferer's family. Once, she says, "I ended up with 11 members of a family in a roundtable conversation." That support helped the man decide to get the treatment he needed.

Both Hinkle and Ridge are clearly empathetic, but it's also clear that walking this daily tightrope—trying to persuade the unpersuadable—has taken an emotional toll.

"Sometimes I can't turn it off," Hinkle says. "I can't go to sleep." She'll lie in bed remembering a woman she talked to that day, thinking, she's probably bathing in Clorox right now.

"You could spend the rest of your life following up on delusory parasitosis," Hinkle warns. And in a sense, she's doing just that. After years of looking for insects that aren't there, "you almost have to laugh, or you'll go crazy."