Millions of American children have been exposed to a parasite that could interfere with their breathing, liver function, eyesight and even intelligence. Yet few scientists have studied the infection in the United States, and most doctors are unaware of it.

The parasites, roundworms of the genus Toxocara, live in the intestines of cats and dogs, especially strays. Microscopic eggs from Toxocara are shed in the animals’ feces, contaminating yards, playgrounds and sandboxes.

These infectious particles cling to the hands of children playing outside. Once swallowed, the eggs soon hatch, releasing larvae that wriggle through the body and, evidence suggests, may even reach the brain, compromising learning and cognition.

The Centers for Disease Control and Prevention periodically tracks positive tests for Toxocara through the National Health and Nutrition Examination Survey. The latest report, published in September in the journal Clinical Infectious Diseases, estimated that about 5 percent of the United States population — or about 16 million people — carry Toxocara antibodies in their blood, a sign they have ingested the eggs.