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. Diagnosis is determined by detection of antibodies to leptospires in the patient's blood, or more rarely by growing the bacteria from blood and other body fluids. Diagnosis is difficult because the bacteria is only found in the blood for 4-5 days. The serologic test for antibodies in the blood must be done at that time.