The differential diagnosis for the pneumonic plague includes disease caused by other biowarfare agents, such as anthrax, tularemia, and melloidosis (glanders), and other pneumonias such as severe community-acquired pneumonia, hantavirus pulmonary syndrome, influenza, or leptospirosis. The septicemic plague has to be differentiated from meningococcemia, Rocky Mountain spotted fever, other gram-negative sepsis, and thrombotic thrombocytopenic purpura.