Leptospirosis is a disease caused by Leptospira bacteria. This bacterium occurs in many different types of animals, including domestic pets, although rats are commonly affected. Leptospirosis is most often transmitted to humans when they come into contact with water or soil that has been contaminated by the urine or other bodily fluids of infected animals. The bacteria can enter the body through the mucous membranes of the eyes, nose or mouth. It can also infect the body through open cuts or broken skin. It generally can not be transmitted from person to person.

Leptospirosis is sometimes classified as a biphasic disease, which means that the illness progresses through two distinct phases. Initial symptoms are similar to many other illnesses and may include acute fever, headache and diarrhea. In some cases, symptoms appear to clear after a period of time but then reappear more severely. This stage of leptospirosis is sometimes called Weil disease and may result in liver or kidney failure, meningitis and hemorrhaging. Leptospirosis is usually treated with antibiotics.

Because the symptoms of leptospirosis are similar to other illnesses, leptospirosis can be difficult to diagnose. Several tests are available, but some of the most accurate tests may take weeks to produce results.

People who work in occupations that involve contact with animals and water (e.g., sewer workers, farmers, veterinarians) are at an increased risk of contracting leptospirosis. Leptospirosis is also a risk for people who participate in sports and other activities that take place in fresh water. Therefore, one of the most effective prevention methods is to avoid contact with potentially contaminated water and to have family pets vaccinated against the disease. There are no human vaccines to prevent infection with leptospirosis.