Emerging and re-emerging viruses continue to surface all over the world. Some of these viruses have the potential for rapid and global spread with high morbidity and mortality, such as the SARS coronavirus outbreak. It is extremely urgent and important to identify a novel virus near-instantaneously to develop an active preventive and/or control strategy. As a culture-independent approach, viral metagenomics has been widely used to investigate highly divergent and completely new viruses in humans, animals, and even environmental samples in the past decade. A new model of Koch's postulates, named the metagenomic Koch's postulates, has provided guidance for the study of the pathogenicity of novel viruses. This review explains the viral metagenomics strategy for virus discovery and describes viruses discovered in human feces in the past 10 years using this approach. This review also addresses issues related to the metagenomic Koch's postulates and the challenges for virus discovery in the future.