The complex interactions between microorganisms and human hosts include the well-known, traditional infectious diseases and the symbiotic relation we have with our normal flora. The media have brought to the public's attention many newly described infectious diseases, such as Ebola virus hemorrhagic fever, that were not part of common medical parlance a decade ago. While flooding us with interesting and often dramatic reports of so-called emerging infectious diseases, the media have largely ignored a more fundamental change in our appreciation of human-microorganism interactions: the discovery that transmissible agents may play important roles in diseases not suspected of being infectious in origin. A well-known example is ulcer disease; other examples include neurodegenerative disease, inflammatory disease, and cancer. These fascinating instances of host-pathogen interaction open new prospects for the prevention of disease through immunization.