In sheer numbers, bacterial cells in the body outnumber our own by a factor of 10, with 50 trillion bacteria living in the digestive system alone, where they’ve remained largely unstudied until the last decade. As scientists learn more about them, they’re beginning to chart the complex symbiosis between the tiny bugs and our health.

“The microbes that live in the human body are quite ancient,” says NYU Medical Center microbiologist Dr. Martin Blaser, a pioneer in gut microbe research. “They’ve been selected (through evolution) because they help us.” And it now appears that our daily antibacterial regimens are disrupting a balance that once protected humans from health problems, especially allergies and malfunctioning immune responses.