We are surrounded by bacteria and their viruses, and those microbes have evolved alongside us. The human microbiome projects around the world seek to explore the microbes in both healthy people and those with diseases. By sampling different body sites and different time intervals we can identify changes in the microbiota that may correspond to different diseases. Despite hundreds of years of culturing bacteria from people, many of the sequences in the human microbiome do not map to any known organisms, and remain unidenitfiable. Comparing samples from different people living in different places around the world using a technique called Cross Assembly, resulted in the identification of a complete new phage that has never been cultured before. We introduce crAssphage, a ubiquitous intestinal inhabitant that is present in approximately half of the people in the world, and accounts for almost 90% of the data in some of the experiments. Working with collaborators worldwide, we found that that phage is present on every continent and has spread where ever humans travel. The virus and the bacteria this virus infects have been evolving with humans for millenia. This ~100kb Bacteroidetes phage is a previously unknown player in the ecology of human intestines and may be responsible for the regulation of growth of Bacterides and its role in our health and disease.