A doctor opened up a can of worms when he looked inside a woman's eye and found a live white worm wiggling inside.

In the horrifying video, uploaded by user Zerofuxgiven on LiveLeak.com, the doctor looks at the woman’s eye through a microscope, and finds a small, white worm twisting and writhing. The worm, identified as a nematode (roundworm), is first seen moving around in the bottom half of her iris before traveling over her pupil. The woman, who doesn’t look like she’s in pain, blinks occasionally as the doctor continues to probe around the eye.

Eye infections from nematodes — there are more than 15,000 known species — are a major cause of blindness in many parts of the world. Worms found in the human eye usually get there after eating meat that isn't sufficiently cooked, after making contact with feces, or after drinking water contaminated with feces from humans or animals. The majority of nematodes are transmitted by insects; if a person is bitten, the larvae of worms penetrate the skin after the bite. They grow into adults, which make their own young, and some of these then travel through the host's tissue, ending up in the human eye.

It can “live in the eye because as a parasite, it has evolved to live inside other animals and is very good at it," Mark Viney, a biologist specializing in nematode roundworms at the University of Bristol, told the DailyMail. “The worm has evolved to change the host’s immune response to further its own survival. The worms secrete molecules, which go and interact with other molecules in the host’s immune system and alter it.”

Nematode infections can be treated with antiparasitic drugs and surgery. According to the LiveLeak video description, the woman did receive treatment for her eye infection. It involved slicing the conjunctiva and draining the aqueous humor with a vacuum tube. “Not pleasant to look at,” the user wrote.