It appears that many commonly recognized syndromes such as the ARDS may well be caused by agents that have only recently emerged as respiratory pathogens. HPS represents one such entity. It appears likely that the increasing pressure of antibiotic use as well as the reemergence of certain pathogens will continue to challenge the clinician. Paramount to the identification and treatment of unusual pneumonias will be the degree with which an effort is made to make an etiologic diagnosis through sputum examination, transtracheal aspirate, bronchoscopy, or lung biopsy. Although pneumococcal pneumonia is the most common community-acquired pneumonia seen by practicing physicians, in all likelihood from time to time a physician will encounter pneumonia caused by one of the unusual pathogens described in this article or else by an altogether new pathogen.