Since the fall 2001 distribution of letters containing Bacillus anthracis spores via the US postal system, increased attention has been paid to the evaluation and management of bioterrorism-related illness. Similarly, the emergence of Severe Acute Respiratory Syndrome (SARS) and other infectious diseases, along with the ongoing threat of global influenza pandemics, have fostered intensive interest in the evaluation and management of emerging infections both in the US and internationally. Data to this end are often limited, however, in that while many such diseases exist in nature, human illness may only rarely occur. The primary purpose of this protocol is to apply standardized, documented, and carefully monitored evaluation and prophylactic and treatment measures in the event of suspected or confirmed exposure to the causative agent of any disease of bioterrorism concern (whether of natural or deliberate origin), to agents under study in biodefense-related research laboratories, or to emerging infectious disease pathogens. Other goals include the characterization of natural history and clinical course of such diseases via the evaluation of close contacts of potentially exposed persons, as well as those who have recovered from illness; the elucidation of the pathophysiology of such diseases; the characterization of immune responses to such diseases; and the evaluation of diagnostic tests for the rapid identification of the causative agents of such diseases in clinical specimens. Further knowledge about diseases of bioterrorism concern and emerging infectious diseases may lead to more effective forms of therapy and improve disease-related morbidity and mortality.