The basic reproduction number (R 0), also called the basic reproduction ratio or rate or the basic reproductive rate, is an epidemiologic metric used to describe the contagiousness or transmissibility of infectious agents. R 0 is affected by numerous biological, sociobehavioral, and environmental factors that govern pathogen transmission and, therefore, is usually estimated with various types of complex mathematical models, which make R 0 easily misrepresented, misinterpreted, and misapplied. R 0 is not a biological constant for a pathogen, a rate over time, or a measure of disease severity, and R 0 cannot be modified through vaccination campaigns. R 0 is rarely measured directly, and modeled R 0 values are dependent on model structures and assumptions. Some R 0 values reported in the scientific literature are likely obsolete. R 0 must be estimated, reported, and applied with great caution because this basic metric is far from simple.