Medieval Christians often feared that Jews were not bound by the same moral rules as Christians. Their helplessness in the face of disease or medical hocus-pocus made them suspicious that their medicine might be poison.
Leprosy had already established medicine as an adjunct of the legal profession: there were court battles over whether a man did or did not have leprosy, because if he was certified to have it, he had to die legally. The city enforced his writing a will and vanishing from society. Some men hired their own doctors and took the city to court. Sometimes juries had to decide if they could trust the word of a Jewish doctor who disagreed with Christian doctors.
All medieval doctors were liable to being sued for malpractice. Doctors were frequently fined, and sometimes they were condemned to banishment or even death. Jewish doctors were not more liable than others, but when they were found guilty, it reinforced Christian suspicions against their whole community. When fines were too heavy for an individual doctor to pay, the Jewish community often helped because paying the fine made the talk settle down.
Jews avoided working as pharmacists. That was the quickest way to be accused of maliciously compounding a poison. If a Christian did the compounding, then the Jewish doctor at least did not bear responsibility if he could prove that his prescription was reasonable. After the plague of 1347-50 sharpened Christian fears that some disease was deliberate Jewish poisoning, Jewish doctors were required to taste their medicinal wines before administering them—and the wine not being kosher was excluded as an excuse.
But the basic problem was that medieval people had begun to expect doctors to care for them in sickness, even if they were not aristocrats, and there was a shortage of Christian doctors. University education was incredibly expensive, and the Church forbade its priests and monks from studying medicine. But Jewish families that arranged for training within a family network were able to manage the training more easily. The number of Jewish doctors was rising, so forbidding Jews too widely would mean cutting off medical care.
Eventually, some rulers forbade Jews to practice medicine. A number of doctors had been accused and convicted of deliberately poisoning their patients; those convicted were hanged. Christians also had rumors going back to to the 12th century that suggested Jews liked to dig up dead Christian bodies and make potions out of their ground-up bones, adding Hebrew spells that invoked Satan.
Provence forbade Jews to practice medicine, or Christians to consult them, in 1306. Sicily did the same in 1310. In 1337, the Pope at Avignon spelled out rules forbidding Christians to consult Jewish doctors unless the patients was at death’s door and nobody else was reachable. But Provence had to revoke its decree a few years later, as did the Pope.
For the next few centuries, this pattern continued. A ruler would forbid Jewish doctors, but everyone would circumvent and complain against these laws. Sometimes the decrees were repealed, other times both the Church and secular officials continued to call in Jewish doctors. It got worse after the plague, and by the early 1400s, Italian cities that had public hospitals were confronted with firing their Jews or dealing with paranoid patients. Still, Christians really never stopped consulting their favorite Jewish doctors. By this time, medicine was widely taught among Jewish families. It was a life-preserver.