Leptospirosis is a zoonosis of worldwide distribution caused by infection with Leptospira interrogans, a pathogenic spirochete. The most important reservoirs are rodents, predominantly rats. Urinary shedding of organisms from infected animals is the most significant source of Leptospira spp. The majority of patients manifest a mild, anicteric febrile illness, but a minority of patients develop a severe form with multiorgan involvement, called Weil's disease. Weil's disease is characterized by multisystem dysfunction and can present with high fever, significant jaundice, renal failure, hepatic necrosis, pulmonary involvement, cardiovascular collapse, neurologic changes and hemorrhagic diathesis.