Shigellosis remains a major public health problem in developing countries. In these nations, the disease affects young children for the most part. The infecting organism causes illness by invading the colonic mucosa. It is closely related to nonpathogenic Escherichia coli, and genetic material can be transferred from one organism to the other, a process increasing the pathogenic potential of the E. coli or reducing the virulence of the strain of Shigella. Knowledge of the genetics of virulence of shigellae enables the construction of living, attenuated oral vaccines that may offer a practical means of controlling the disease.