Leptospirosis is a type of bacterial infection spread by animals. It's caused by a strain of bacteria called leptospira.In 90% of cases, leptospirosis only causes mild flu-like symptoms, such as a headache, chills and muscle pain. However, in some cases the infection is more severe and can cause life-threatening problems, including organ failure and internal bleeding.t's caused by a strain of bacteria called leptospira. In 90% of cases, leptospirosis only causes mild flu-like symptoms, such as a headache, chills and muscle pain. However, in some cases the infection is more severe and can cause life-threatening problems, including organ failure and internal bleeding.