Weilâs Disease Overview. Weilâs disease is a severe form of a bacterial infection known as leptospirosis. Leptospirosis is caused by bacteria from the genus Leptospira. The infection typically only causes mild flu-like symptoms, including headaches and chills.In severe cases, such as in Weilâs disease, it can lead to organ failure and bleeding.t is also possible to become infected through rodent bites and drinking contaminated water. The mild form of the disease is known as leptospirosis. If the bacteria infect other organs of the body, such as the kidneys, heart, liver, lungs, or brain, it can lead to a more severe disease.