No one has seen crAssphage under the microscope, but we know what its genome looks like—Bas Dutilh from Radboud University Medical Centre pieced it together using fragments of DNA from the stools of 12 individuals. He found crAssphage in all of them. Then, he found it in hundreds more.

To study the microbes that live in a person’s guts, scientists will typically collect a stool sample, break all the DNA within into small fragments, and sequence these pieces. The result is a metagenome: a mish-mashed collection of DNA from all the local bacteria, viruses and other microbes.

Dutilh’s team, led by Rob Edwards at San Diego State University, analysed 466 metagenomes that have been added to public databases and found