Probiotics are quickly becoming a huge business, claiming to heal millions with the power of balancing the biosphere of symbiotic bacteria in their guts. But the science is a lot more complicated than hawkers of supplements and yogurts will have you believe, and far from helping good bacteria recolonize your digestive tract, they’re merely delivery mechanisms for benign and temporary microscopic visitors. So where does the science end and hype begin? Are there any real benefits to probiotics? And what can we learn from studying our bacterial biomes?