Leptospires can survive in moist conditions outside the host for many days or even weeks. Humans can be infected by other animals. Direct or person to person spread is exceptionally rare. The bacteria enter through skin abrasions or through the eyes, nose and mouth. Human infection can range from a mild influenza-like illness to meningitis or kidney failure, severe pains in calf and back muscles, red eyes and prostration, fever, headache, chills, conjunctival suffusion, and less frequently by rash, jaundice, or renal insufficiency.
Some cases develop abdominal pains with diarrhoea and vomiting.
A fatal outcome is virtually unknown in patients in whom jaundice is not a feature. In those cases where jaundice occurs the illness is know as Weil's disease. Human infection is more common in men than women. An incubation period of 7 to 12 days is usual. Most cases present with an influenza-like illness which resolves in two or three weeks.