A bacterial disease caused by a Leptospira species that is now recognized as one of the new, emerging infectious diseases. The Centers for Disease Control and Prevention (CDC) has referred to “new, reemerging, or drug-resistant infections whose incidence in humans has increased within the past two decades or whose incidence threatens to increase in the near future.” Leptospirosis, presumed to be the most widespread zoonosis (a disease of animals that may be transmitted to humans) in the world, is characterized by a broad spectrum of clinical manifestations, varying from a subclinical infection (showing no symptoms) to a fatal disease.