In Flat Black most people have bacteria living in their mouth, stomach, and different parts of their gut, on different parts of their skin etc. that were designed by genetic engineers for different advantages. The custom is of such antiquity that even people on the most backward colonies have skin and gut flora descended from the work of some long-defunct genetic engineer of Old Earth.
Perhaps the most obvious role of these cultures is in modifying the scent of the skin and breath. They crowd out wild types and produce pleasant scents (or, in people who choose wisely) no scent at all. Another fairly obvious advantage is that oral flora crowd out wild types and do not cause tooth decay (they do not not produce lactic acid as a metabolite). Designer skin floras do not cause acne. Designer gut floras produce useful amounts of B-group and other vitamins.
Almost all of the original designer cultures did not originally cause disease even in immune-compromised people, and even when they got into places they shouldn't be, such as the bloodstream and peritoneal cavity. But bacterial evolution is rapid enough that many of these bacterial species have evolved into pathogenic niches.
