Leptospirosis is a bacterial disease that affects humans and animals. It is caused by bacteria of the genus Leptospira.In humans, it can cause a wide range of symptoms, some of which may be mistaken for other diseases. Some infected persons, however, may have no symptoms at all.t is caused by bacteria of the genus Leptospira. In humans, it can cause a wide range of symptoms, some of which may be mistaken for other diseases. Some infected persons, however, may have no symptoms at all.