We may be missing the key to one of the biggest boons to public health since the introduction of iodine into the food supply in 1924.

Scientists at the University of California, Davis, have found that a strain of bacteria called B. infantis that is thought to have been the dominant bacterium in the infant gut for all of human history is disappearing from the Western world. According to their research, this was probably caused by the rise in cesarean births, the overuse of antibiotics and the use of infant formula in place of breast milk.

Indeed, nine out of 10 American babies don’t harbor this bacterium in their gut, while researchers suspect that the majority of infants in less industrialized countries do.

Bruce German, a professor of food science and technology and one of the U.C. Davis researchers, says, “The central benefits of having a microbiota dominated by B. infantis is that it crowds all the other guys out” — especially pathogenic bacteria, which can cause both acute illnesses and chronic inflammation that leads to disease.