In terms of sheer numbers, rod-shaped bacteria dominate the field of pathogenic bacteriology. Included among the diseases caused by bacilli are ancient, deadly, and fascinating ones such as bubonic plague, typhoid fever, tetanus, and leprosy, as well as newly emerging ones such as listeriosis and Legionnaires disease. In chapters 19 and 20, we will characterize and differentiate genera and medically important species using a simple modification of a system presented in Bergey's Manual of Systematic Bacteriology. This system divides the bacilli by Gram reaction and subgroups them by morphological characteristics and oxygen utilization (see figure E, page 542). Each section covers unique features of the pathogens and the epidemiology, pathology, and control measures of the principal diseases.