The urban built environment is a prime setting for microbial transmission, because just as cities serve as hubs for migration and international travel, components of the urban built environment serve as hubs that drive the transmission of infectious disease pathogens. The risk of infectious diseases for many people living in slums is further compounded by their poverty and their surrounding physical and social environment, which is often overcrowded, is prone to physical hazards, and lacks adequate or secure housing and basic infrastructure, including water, sanitation, or hygiene services. To examine the role of the urban built environment in the emergence and reemergence of infectious diseases that affect human health, the National Academies of Sciences, Engineering, and Medicine planned a public workshop. This publication summarizes the presentations and discussions from the workshop.