Anibiotics are wildly over used in children. It's not uncommon for someone to reach age 40 and have taken 30 courses of anibiotics. We lose hundreds of microb species, while some of those species did really important stuff for us. The real kiker here is unhealthy microb species like ecoli and clebsial and central bactrand seudamars. These are stool species max poliferate that ascend into the 24 feet of small bowl ilium jajunum dadinum stomach. They add up to 30 feet of unhealthy, mostly stul microbes but when that happens they only live hours to days. So there's massive turnover of trillions of microbes