A healthy person has a general balance of good and bad bacteria. But that balance is thrown off when someone gets sick. So, to help boost their levels of good bacteria, many people take probiotic supplements — live bacteria inside of a pill. Various commercial probiotic supplements are available for consumer purchase, and while health experts generally agree about their overall safety, controversy surrounds their efficacy.

Inside the human body lives a large microscopic community called the microbiome, where trillions of bacteria engage in a constant "tug of war" to maintain optimal levels of good and bad bacteria. Most of this struggle takes place within the body’s gastrointestinal tract, as bacteria help with digesting food and support the immune system. Although health experts believe good "gut" health is key to a person’s health and well-being, scientists are still developing a detailed picture of what goes on inside a person’s gastrointestinal tract.