The Black Death was a dreadful disease that came from Asia. It was probably started by a bacillus or germ, found in the stomach of a certain kind of flea. This flea lived on black rats, which came to Europe on trading ships or caravans from the East. A bite from one of these fleas could infect a human. Then, as the disease spread, one person could infect others just by being in contact with them. Doctors had no idea what to do. They did not understand the cause of the disease either. About one fifth of all the people in Europe died. Eventually the plague died down, but it came back in several smaller outbreaks over the next 300 years.

Physicians relied on crude and unsophisticated techniques such as bloodletting and boil-lancing (practices that were dangerous as well as unsanitary) and superstitious practices such as burning aromatic herbs and bathing in rosewater or vinegar.

Meanwhile, in a panic, healthy people did all they could to avoid the sick. Doctors refused to see patients, priests refused to administer last rites, and shopkeepers closed their stores. Many people fled the cities for the countryside, but even there they could not escape the disease: It affected cows, sheep, goats, pigs and chickens as well as people. In fact, so many sheep died that one of the consequences of the Black Death was a European wool shortage.