Abstract: Infectious diseases caused by pathogenic microorganisms resident in the alimentary tract of humans and animals can be prevented and treated by administering to the alimentary tract of the human or animal an effective amount of a composition containing at least one lectin capable of binding to an infective microorganism and diminishing its infective capability of the microorganism. The lectin is administered dispensed in a pharmaceutically acceptable non-toxic vehicle. Peptic ulcer disease caused by infection with H. pylori can be treated by oral administration of lectins that bind to the pathogen. A beneficial ecology of H. pylori can be maintained in infected patients by chronic oral administration of lectins that bind to the pathogen.