The infection passes from animals (commonly rats, cattle, pigs and dogs) to humans. In most cases, leptospirosis causes a mild illness. But, in some people, a more severe illness occurs. This more severe form is commonly referred to as Weil's disease.The main treatment for leptospirosis is antibiotics and most people make a full recovery.n most cases, leptospirosis causes a mild illness. But, in some people, a more severe illness occurs. This more severe form is commonly referred to as Weil's disease. The main treatment for leptospirosis is antibiotics and most people make a full recovery.