The human body carries trillions of bacteria, viruses, and other tiny microbes. Scientists call these resident bugs and all of their genes the “human microbiome.”

Some of these microbes can be harmful, but most have lived and evolved with humans for millennia. They help train our immune systems to attack only foreign invaders, not our own joints and organs, and they pull essential nutrients out of the food we eat.

Find out why people in the Western world have far fewer kinds of these helpful bacteria than their cousins in developing countries. And see how that might be connected to rising rates of autoimmune diseases, such as asthma, and metabolic ones, such as obesity and type 2 diabetes.

Read Part 1 of our series here.

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