As currently available antibiotics become ineffective due to the rise in antibiotic resistance among pathogenic bacteria, development of completely new classes of antibiotics is critical. Classic antibiotics target pathogens and commensal bacteria indiscriminately; therefore, their use puts selective pressure on both populations. Because of the abundance of commensals within a mammalian host, antibiotic resistance is thought to arise more frequently in commensal bacteria and is horizontally transferred to pathogens. In contrast to classic antibiotics, virulence blockers are compounds that selectively inhibit the expression or function of a virulence factor in a pathogen or group of pathogens. Advantages of virulence blockers are twofold. For one, selective pressure on a limited number of microbes, i.e., only pathogens expressing the molecular target of the virulence blocker, should limit the evolution of resistance. Second, the decreased commensal killing by virulence blockers has the potential to preserve a healthy microbiota, which is critical for maintaining gut homeostasis and defending against opportunistic pathogens. Type III secretion systems (T3SS) are bacterial appendages required by dozens of pathogens to cause disease, including Salmonella, enteropathogenic Escherichia coli (EPEC), Shigella, Pseudomonas, and Yersinia, but they are largely absent in nonpathogenic bacteria. Bacteria use T3SS to inject bacterial effector proteins into target host cells to manipulate host processes for the benefit of the pathogen. Seven T3SS injectisome families have been identified and share a number of homologous membrane-associated components with the flagellar basal body. Agents that target T3SS would be key virulance blockers for a set of pathogens that are very important to human and animal health as are methods of screening for such agents.