Of all the naturally occurring poisons, mercury has perhaps been the most actively employed in civilised life. It was – and still is – everywhere we look, in measuring instruments such as thermometers and barometers, in electric apparatus and lighting, in our mended teeth and in mirrors. It will be a long time before we can live in a mercury-free world. It was also very actively employed for centuries in attempting to cure syphilis and the pox. The treatment was often more painful than the illness, and it remained questionable whether the use of mercury created a disease rather than cured it. This continued almost to the mid-twentieth century. Andrew Cunningham explores this persistent reliance on, and faith in, the restorative properties of mercury: a known poison used against another poison, the virus of the pox.