Leptospirosis is a zoonosis found worldwide, the main reservoir of which is the rat. Human infection generally results from exposure to contaminated river or lake water or animals. Around 600 cases are diagnosed per year in France. Half of these cases occur in French overseas territories, where the incidence can be more than 100 times higher than in mainland France. Leptospirosis has been under-diagnosed because of non-specific symptoms, inadequate surveillance system, and lack of readily available quick and simple diagnostic tests. Most cases of leptospirosis are currently detected by PCR amplification of bacterial DNA from the blood during the first week after the onset of symptoms, or by detection of antibodies during the second week of the disease. More than 300 serovars have been identified among leptospires, including serovar Icterohaemorrhagiae, the most frequent in human infections. Leptospirosis remains a major public health issue in many developing countries, one century after discovering the causative agent. Leptospirosis is expected to become more important due to a rapid urbanization in developing countries (slums), global warming, and extreme climatic events (floods).