A hundred trillion bacteria, viruses, and other microbes—collectively known as the human microbiome—live on and inside our bodies. In 2012 scientists on two big projects conducted the most comprehensive census so far of this inner world.

The U.S.-based Human Microbiome Project used genomic analysis to I.D. microbes in the noses, gums, tonsils, genital tracts and guts of more than 200 Americans. In parallel, a European project called MetaHIT studied 124 Europeans. Both projects found that each person’s body supports many different microbial ecosystems, and that our microbiomes vary greatly from person to person.

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