Whether they make you fat, fart, or freak out, microbes play a central role in your life. Right beneath your nose—on your face, in your gut, and everywhere in between—trillions of bacteria, viruses, and fungi are so abundant in your body, they outnumber your human cells. But these aren’t just nasty hitch-hikers. Many are crucial to your survival. Evidence suggests that a diverse microbiome can keep you healthy and, conversely, a damaged one could kill you. NOVA Wonders peers into this microscopic world to discover the fascinating, bizarre, and downright surprising secrets of the human microbiome, including the world’s largest stool bank, which transforms raw stool into life-saving poop pills.