What are probiotics? A beginner's guide to gut health

  The human gastrointestinal tract houses trillions and trillions of microbial cells—not millions, not billions, trillions. It’s also home to several different kinds of microorganisms, from bacteria and viruses to fungi, archaea, and protozoa. In other words, there’s a lot going on—and a delicate equilibrium to maintain. Keeping that balance isn’t quite as simple as