latest estimate, the human body contains about 38 trillion bacterial cells and approximately 30 trillion human cells. Scientists know that this community of gut bacteria, or microbiota, plays a role in healthy digestion and immune function.
This has spurred sales of probiotics that manufacturers promote as boosting populations of “friendly” bacteria. Many of these products’ supposed health benefits remain unproven, however. Studies have nonetheless found strong associations between particular species of gut bacteria and a range of diseases, including obesity, inflammatory bowel disease (IBD), and colorectal cancer. Fecal transplants from healthy individuals can restore populations of beneficial gut bacteria and displace disease-causing.