Most people are infected through contact with contaminated soil or water during outdoor activities.
Fever, headache, and other symptoms occur in two phases, separated by a few days.
A severe, potentially fatal form damages many organs, including the liver and kidneys.
Detecting antibodies against the bacteria in blood or identifying the bacteria in a sample taken from infected tissue confirms the diagnosis.
Infections are treated with antibiotics and, if severe, sometimes with fluids given intravenously and dialysis.

(See also Overview of Bacteria.)
Leptospirosis occurs in many wild and domestic animals, including rats, dogs, and farm animals such as cattle, horses, sheep, goats and pigs. Some animals act as carriers and pass the bacteria in their urine. Others become ill and die. People acquire these infections directly through contact with infected animals or indirectly through soil or water contaminated by urine from an infected animal.
Leptospirosis is an occupational disease of farmers and sewer and slaughterhouse workers. However, in the United States, most people become infected during outdoor activities when they come in contact with contaminated soil or fresh water, particularly while swimming or wading. Outside the United States, outbreaks have occurred after heavy rainfall or freshwater flooding. Leptospira can survive for several weeks to months in sources of fresh water (such as lakes and ponds). However, they can survive for only a few hours in salt water.
The 40 to 100 infections reported every year in the United States occur mainly in the late summer and early fall. Because mild leptospirosis typically causes vague, flu-like symptoms that go away on their own, many infections are probably unreported.