Microbes are organisms that cannot be seen with the naked eye. They include bacteria, archaea, protists, viruses, prions, and fungi. They account for most of the diversity of life on Earth. Far more microbes inhabit a single person’s body than there are people on the entire planet. Microbes also dominate in terms of their ability to live and thrive in extreme environments, including clouds, deep-sea volcanoes, springs, ice caps, and animal intestines.

Most animals, including humans, exhibit numerous associations with microbes. Our "human microbiome" includes our skin, hair, mouth, and gut.

Humans typically associate microbes with disease. Most of our human microbiome, however, is neutral or even beneficial. Microbes help digest food, absorb nutrients, and out-compete harmful bacteria in the intestines. They produce vitamins and proteins that human genes cannot produce. They prevent the growth of harmful skin bacteria and further aid the immune system in fighting infections and diseases throughout the human body.