In a recent study at Rice University, published in PLOS Genetics, scientists found the first direct link between a diet low in vitamin B12 and an increased risk of infection by two potentially deadly pathogens. And they did it using roundworms.

Why? Although the worms are one of earth’s simplest animals, they share a trait with humans: They cannot make B12 and must get all they need from their diets.

About 10 percent of U.S. adults get too little B12 in their diets—the vitamin is most prevalent in animal foods, including clams, liver and kidneys, and certain fish like trout, salmon and tuna. Vegetarians and vegans, as well as people older than 50 (the ability to absorb B12 decreases with age), should thus get the vitamin from fortified foods or dietary supplements.