WE know that the proteins called gluten, found in wheat and other grains, provoke celiac disease. And we know how to treat the illness: a gluten-free diet. But the rapidly increasing prevalence of celiac disease, which has quadrupled in the United States in just 50 years, is still mystifying.

Scientists are pursuing some intriguing possibilities. One is that breast-feeding may protect against the disease. Another is that we have neglected the teeming ecosystem of microbes in the gut — bacteria that may determine whether the immune system treats gluten as food or as a deadly invader.

Celiac disease is generally considered an autoimmune disorder. The name celiac derives from the Greek word for “hollow,” as in bowels. Gluten proteins in wheat, barley and rye prompt the body to turn on itself and attack the small intestine. Complications range from diarrhea and anemia to osteoporosis and, in extreme cases, lymphoma. Some important exceptions notwithstanding, the prevalence of celiac disease is estimated to range between 0.6 and 1 percent of the world’s population.

Nearly everyone with celiac disease has one of two versions of a cellular receptor called the human leukocyte antigen, or H.L.A. These receptors, the thinking goes, naturally increase carriers’ immune response to gluten.