Picture a boat hull covered with barnacles, a dried lotus seed pod, milk bubbles on a latte, or a honeycomb. Images of these objects are harmless—unless you're one of the millions of people suffering from trypophobia. Then they're likely to induce intense disgust, nausea, and fear, and make your skin crawl.

Coined fairly recently, the term trypophobia describes the fear of clusters of holes. The phobia isn’t recognized by the Diagnostic and Statistical Manual of Mental Disorders, but its visibility on the internet suggests that for many, it’s very real. Now, scientists in the UK think they've pinpointed the evolutionary mechanism behind the reaction.

Tom Kupfer of the University of Kent and An T. D. Le of the University of Essex shared their findings in the journal Cognition and Emotion. According to their research, trypophobia evolved as a way to avoid infectious disease. Thousands of years ago, if you saw a person covered in boils or a body covered in flies, a natural aversion to the sight would have helped you avoid catching whatever they had.

But being disgusted by skin riddled with pathogens or parasites alone doesn't mean you're trypophobic; after all, keeping your distance from potential infection is smart. But trypophobia seems to misplace that reaction, as the authors write: "Trypophobia may be an exaggerated and overgeneralized version of this normally adaptive response."

Lotus seed pods are a common trigger of trypophobia.Vmenkov, Wikimedia Commons // CC BY-SA 3.0

This explanation is not entirely new, but until now little research has been done into whether it's accurate. To test their hypothesis, the scientists recruited 376 self-described trypophobes from online forums, and another 304 college students who didn't claim to have the affliction. Both groups were shown two sets of images: The first depicted clusters of circle-shaped marks on animals and human body parts (the "disease-relevant cluster images"); the second showed clusters of holes on inanimate objects like bricks and flower pods ("disease-irrelevant cluster images"). While both groups reported feeling repulsed by the first collection of photographs, only the trypophobes felt the same about the pictures that had nothing to do with infection.

Another takeaway from the study is that trypophobia is more related to sensations of disgust than fear. This sets it apart from more common phobias like arachnophobia (fear of spiders) or acrophobia (fear of heights). And you don't have to be trypophobic to be disgusted by a video of Suriname toadlets being born through holes in their mother's back. We can all be grossed out by that.