Leptospirosis (also known as fall fever and mud fever) is a bacterial infection caused by any of the approximately 200 serotypes of the Leptospira spirochete. It is carried primarily by amphibians, reptiles, and some mammals (eg, dogs, skunks, raccoons, rats, swine, and cattle), both wild and domesticated. Leptospirosis may be passed to humans by direct contact with or aerosol inhalation of contaminated urine or body fluids. As an agent of bioterrorism, the most likely method of dispersal would be as an aerosol. While naturally acquired infection can present in a myriad of ways – from subclinical to severe (overall mortality 1%-5%) – the large initial exposure following a bioterrorism attack would likely tend to produce more severe illnesses. The incubation period of leptospirosis is usually 5-14 days, although it can range anywhere from 2-30 days. The most commonly recognized syndrome includes the onset of nonspecific symptoms: fever, chills, headache, nausea and vomiting, and transient rash (eg, pretibial erythematous papules or mucosal petechiae). A septicemic phase involving the above symptoms plus myalgias (of calf and low back, 80%) and conjunctivitis can occur, usually followed by an immune-mediated phase: fever, aseptic meningitis, uveitis, jaundice, adenopathy, renal insufficiency, polyuria, polydipsia, myalgias, and purpura. The illness may progress to Weil's disease (mortality about 40%), the symptoms of which include jaundice, kidney and/or liver failure, meningitis, pneumonitis with hemoptysis, ARDS, hemorrhage, shock, and death.

Humans can also become infected if water or soil contaminated with infected urine or animal tissue comes in contact with breaks in the skin or mucosal surfaces. This can occur through recreational or occupational exposures. People at risk for leptospirosis include military personnel, veterinarians, dog and other pet owners, slaughterhouse and sewer workers, campers, swimmers, waders, white water rafters, and dairy, cattle and pig farmers. The infection can also be transmitted person-to-person by direct contact with contaminated urine and body fluids.

Transmission tends to be greatest during warm and wet seasons, typically fall in North America.

Cases of leptospirosis have been documented on all continents in both temperate and tropical climates. However, few cases occur in the United States (100-200 annually, of which half occur in Hawaii).

There is no vaccine against leptospirosis in North America, although a Chinese vaccine for rice paddy workers exists, but protection is short-lived, and boosters are required.