During the pandemic's peak in Europe, from 1348 to 1351, more than 200 Jewish communities were wiped out, their inhabitants accused of spreading contagion or poisoning wells.
The swine flu outbreak of 2009 has been nowhere near as virulent, and neither has the reaction. But, as in pandemics throughout history, someone got the blame - at first Mexico, with attacks on Mexicans in other countries and calls from American politicians to close the border.
In May, a Mexican soccer player who said he was called a "leper" by a Chilean opponent spat on his tormentor; Chilean news media accused him of germ warfare. In June, Argentines stoned Chilean buses, saying they were importing disease. When Argentina's caseload soared, European countries warned their citizens against visiting it.

"When disease strikes and humans suffer," said Dr Liise-anne Pirofski, chief of infectious diseases at Albert Einstein College of Medicine and an expert on the history of epidemics, "the need to understand why is very powerful. And, unfortunately, identification of a scapegoat is sometimes inevitable."
A recent exhibition, "The Erfurt Treasure," at the Yeshiva University Museum in Manhattan, displayed a timely and depressing memento of this all too human habit. A chest with more than 600 pieces of gold jewelry, including a magnificent 14th-century wedding ring, was dug up during excavations in what was once a thriving Jewish quarter in Erfurt, Germany. It also held 3,141 silver coins, most with royal portraits; the last king depicted on them died in 1350.

That, said Gabriel M Goldstein, the museum's associate director of exhibitions, strongly suggests the hoard was buried in 1349, the year the plague reached Erfurt.
"Why put such a huge investment portfolio in the ground and leave it for 700 years?" he asked. "There was a major uprising against Erfurt's Jews -- records say 100 or 1,000 were killed. Seemingly, whoever hid it died and never came back."
Dr Martin J Blaser, a historian who is chairman of medicine at New York University's medical school, offers an intriguing hypothesis for why Jews became scapegoats in the Black Death: they were largely spared, in comparison with other groups, because grain was removed from their houses for Passover, discouraging the rats that spread the disease. The plague peaked in spring, around Passover.
But in every pandemic, the chain of causation is intricate. Historian William H McNeill, author of Plagues and Peoples, suggests that ultimate blame may rest with Möngke Khan, grandson of Genghis, who in 1252 sent his armies as far south as present-day Burma, putting them in contact with rodents whose fleas played host to yersinia pestis, the plague bacillus. After yersinia returned with them to the flea-bitten marmots of the Eurasian steppes, it began creeping through the rodent burrows lining Mongol caravan routes, which stretched as far west as the Black Sea. That's where plague-ridden rats boarded ships in the besieged Crimean port of Kaffa in 1346, taking it to Europe.
But that lets off the hook the Indian or Egyptian sailors who had presumably first moved the wild black rat out of India 1,000 years earlier. And then, who in prehistory does one blame for first carrying yersinia north from its original home in the Great Lakes region of Africa? Manchurian nomads, Dr McNeill said, avoided plague because they believed marmots harbored the souls of their ancestors, so it was taboo to trap them, although shooting them was permitted. But in the early 20th century, trapping by immigrants from China contributed to plague outbreaks.
And Tamils from India working as plantation labourers in Malaysia may have had less malaria and dengue than their Malay and Chinese co-workers did because they never stored water near their houses, leaving mosquitoes no place to breed.
The most visible aspect of blame, of course, is what name a disease gets. The WHO has struggled mightily to avoid the ethnic monikers given the Spanish, Hong Kong and Asian flus, instructing its representatives to shift from “swine flu” to “H1N1” to "A (H1N1) S-O.I.V." (the last four initials stand for "swine-origin influenza virus") to, recently, "Pandemic (H1N1) 2009."
Dr Mirta Roses, director of the Pan American Health Organisation, said that in the pandemic's early days, she fought suggestions that it be named the Mexican flu or the Veracruz flu or the La Gloria flu after the country, state and town where it was discovered.
By the old naming conventions, the 1918 Spanish flu probably ought to be known as the Kansas flu. According to The Great Influenza: The Epic Story of the Deadliest Plague in History, John M Barry's history of the epidemic, the first identifiable cases arose in Haskell County, in Kansas. They soon spread to Fort Riley, from there to other military bases, and then to Europe in troop ships. France, Germany and Britain had war censors controlling news reports; Spain did not. Spain got the blame.
Most human diseases originate in animals. While culling animals sometimes makes sense as a public health measure - for example, culling chickens to stop an outbreak H5N1 avian flu -- animals are also sometimes "punished" pointlessly. In May, the Egyptian government slaughtered thousands of pigs belonging to the Coptic Christian minority, despite international protests that doing so was racist against Copts and medically pointless because the disease was already in people. When the swine flu arrived anyway -- in a 12-year-old American girl, the first confirmed case -- the government vowed to hunt down the last few pigs hidden by poor families and kill them on the spot.
In Afghanistan, Khanzir, the country's only pig, a curiosity in the Kabul Zoo, was quarantined to keep him away from the goats and deer he had formerly eaten with.
And during the spread of the avian flu around Asia, Thailand's government shot open-billed storks in its cities and chopped down the trees they nested in, even though the flu had not been found in a single stork.
Though the truth is that diseases are so complex that pointing blame is useless, simply deflecting blame may be more efficient.
During the Black Death, Pope Clement VI issued an edict, or bull, saying Jews were not at fault. He did not, of course, blaspheme by blaming God. Nor did he blame mankind's sins. That would have comforted the Flagellants, the self-whipping sect who were the bull's real target; they often led the mobs attacking both Jews and the corrupt church hierarchy, and were considered heretics. Nor did it blame Möngke Khan or Yersinia pestis.
It would be 500 years until the "germ theory" of disease developed.  No, the pope picked a target particularly tough to take revenge upon: a misalignment of Mars, Jupiter and Saturn.
The New York Times