Because mercury is poisonous, it can have a detrimental effect on the source of a venereal disease. Of course, it was not immediately recognized that it could also have an ill effect on the patient. The cure could kill even more quickly than the disease. In addition, the "cure" only inhibited the disease rather than actually eliminating it. This gave rise to the popular saying, "A night with Venus, a lifetime with Mercury." The use of mercury is not unlike the early broad spectrum radiations that were used against cancer. Many patients died of the cure as doctors hoped to kill the disease. This is a great website on the history of the disease with a discussion of the role of mercury.