Leptospirosis is caused by the Leptospira bacteria, a type of motile bacteria called a spirochete. The bacteria occur in the urine of animals that have the disease. They survive in water such as streams, ponds and puddles, especially during warmer, rainy seasons. They are also more common in tropical climates where there’s likelihood of standing water or flooding. Rat infestations in densely populated urban areas can also be a factor.