Videos of doctors squeezing patients’ pimples until goo gushes out are a huge hit online—racking up millions of views. Sandra Lee (known as Dr. Pimple Popper) started the craze by posting footage of dermatology procedures. She wanted to show what her job as a doctor who treats skin disorders involves. Lee had no idea that watching close-ups of bursting blemishes would become an internet phenomenon. But why do people find the gag-inducing footage so fascinating?

“Disgusting things capture our attention, and there are good reasons why,” says Daniel Kelly. He’s a professor at Purdue University in Indiana and author of Yuck!—a book about why people find things gross. Feeling disgust is a survival mechanism, says Kelly. “It’s a strong emotional reaction that makes us keep our distance from things that can make us sick. It’s our [disease-fighting] immune system’s first line of defense.” Bodily fluids, such as snot, blood, and pus—like that oozing from a popped zit—trigger disgust because in some instances they might spread illnesses.

Blemishes, though, aren’t harmful to you or anyone else. They’re just pores, or small openings in the skin, clogged with oily sebum, dead skin cells, and bacteria. But watching a doctor treat a pus-filled pimple still grosses people out—a feeling, believe it or not, that some enjoy. “It’s similar to the appeal of scary movies,” says Kelly. “People like to experience the full range of possible emotions, including disgust. Those videos offer emotional thrill without any real danger.”