After seeing a spider or beetle scurry past you, it’s normal to get a creepy-crawly feeling, even if you know there’s nothing on you. For many people, though, the persistent sensation of phantom insects or parasites crawling underneath their skin—known as formication—is very real, Newsweek reports.

The condition is called delusional infestation, and although cases have been documented around the world, there hasn’t been enough research to determine if it’s a skin condition or psychological disorder. However, two new studies are attempting to shed light on the mysterious ailment that can cause symptoms such as itching, fatigue, joint pain, rashes or lesions, and difficulty concentrating. Some people have reported picking “fibers” out of their skin.

Researchers from the Mayo Clinic and Denmark’s Aarhus University Hospital believe tens of thousands of Americans could have this condition, making it more common than previously thought. Their study, published in the journal JAMA Dermatology, found that people with the condition are often “resistant to medical evidence [showing that there is no infestation] and reluctant to pursue psychiatric evaluation.” Some patients, convinced that they have something crawling underneath their skin, self-harm with tweezers, bleach, or razor blades.

The researchers stopped short of calling it a psychological condition, but they did conclude that schizophrenia, dementia, other psychiatric conditions, and drug use can trigger delusional infestation in some cases, Science News reports.

Another new study, published in the journal Annals of the Academy of Medicine of Singapore [PDF], also seemed to favor a psychological explanation for the condition. The researchers noted that Chinese patients with the condition were treated with antipsychotics, and 10 of the 11 patients with isolated cases of delusional infestation (who had no other underlying disorders) improved with medication.

However, other researchers have drawn different conclusions, arguing that the condition is the skin's response to “tick-borne pathogens” typically associated with Lyme disease. The condition has gone by several names over the years, including Morgellons disease—a term coined in 2004 by a medical researcher and mother who says she found “fibers” on her young son’s skin after he kept scratching at the "bugs" he claimed were there. Regardless of the origin, what's clear is that the condition has very real consequences for those who suffer from it, and more research is needed to find suitable treatments.

[h/t Newsweek]