The Committee judged that a vaccine approach to control a number of diseases caused by microorganisms was not attainable within the next 20 years. In some instances not enough information was available concerning the antigens that stimulate protective immune response or the host responses necessary to provide protection. In other cases, a class of infectious agents are known to cause disease (e.g., periodontal disease), but the major contributors to disease within that class are not yet identified. In other cases, knowledge of the natural history of the infection was inadequate. Some of these infectious diseases against which vaccines were not yet considered feasible occur in healthy hosts who experience the loss of integrity of the skin or the disruption of normal intestinal barriers to microorganisms, which permits the development of secondary infections (e.g., infections caused by Clostridium perfringens or Bacteroides fragilis). Other diseases are the result of intimate exposure of healthy hosts to others who harbor certain pathogens (e.g., Treponema pallidum and Mycobacte