Why they called it French is because of a notorious outbreak of it in the French army, but you may know that this phrase referes to the dreaded Syphilis. The disease is thought to have reached Europe from America, via Columbus's ships and the first known epidemic struck Italy in the late 1400s. Interestingly enough it has been called the English disesase by the French, Polish by the Russians, and the Christian disease by the tribes of the Arabian lands.
Many well known figures in history are said to have suffered from suphilis, most notably Guy de Maupassant, Charles VIII of France, Friedrich Nietzsche, Al Capone, Paul Gaugin and even Adolf Hitler. In its late stages the disease causes dementia, whereby the suffered became insane.
Nowadays syphilis is treated with antibiotics but before the Second World War people were very unfortunate indeed if they happened to contract it. It is very much a sexually transmitted disease and therefore when you got it you were instantly stigmatized as being debauched, even if you had acquired it through legitimate relations as in marriage. Unfortunately the effects of the treatment were almost as unpleasant as the symptoms of the disease itself. Doctors in the 18th century would prescribe a course of mercury, which had to be ingested by the patient. As you can imagine, mercury, like lead, is highly toxic. Ingesting mercury, especially at these high dosages, caused mercury poisoning. After a few days of this treatment, the patient would start to salivate uncontrollably and the saliva was black, which must have been very disturbing indeed. Curiously enough, doctors saw this as a good thing and aimed for more salivation, literally poisoning the person more and more. Of course, mercury treatment did relieve the symptoms of syphilis but did not cure it.
Mercury was used to treat syphilis up until the late 1940s, when antibiotics were introduced. Of course people were not made to ingest so much mercury so as to salivate, as in the 18th century, but small dosages of mercury combined with arsenic and bismuth were administered.
The world was radicaly changed by the discovery of antibiotics by Alexander Fleming.
For more info on syphilis, please see link below: