Established infectious agents continue to be a major cause of human morbidity and mortality worldwide. However, the causative agent remains unknown for a wide range of diseases; many of these are suspected to be attributable to yet undiscovered microorganisms. The advent of unbiased high-throughput sequencing and bioinformatics has enabled rapid identification and molecular characterization of known and novel infectious agents in human disease. An exciting era of microbe discovery, now under way, holds great promise for the improvement of global health via the development of antimicrobial therapies, vaccination strategies, targeted public health measures, and probiotic-based preventions and therapies. Here, we review the history of pathogen discovery, discuss improvements and clinical applications for the detection of microbially associated diseases, and explore the challenges and strategies for establishing causation in human disease.