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In the latest exploration into the universe of organisms inhabiting our bodies, microbiologists have discovered new viral genes in faeces. They find that the composition of virus populations inhabiting the tail ends of healthy intestines (as represented in our stools) is unique to each individual and stable over time. Even identical twins — who share many of the same intestinal bacteria — differed in their gut's viral make-up.

More than 80% of the viral genetic sequences found, which included sequences characteristic of both animal and bacterial viruses, have never been reported previously. "This is a largely unexplored world," says Jeffrey Gordon at Washington University in St Louis, Missouri, and an author on the paper, which is published in Nature today1. "We are truly distinct lifeforms — sums of microbial and human parts."