No one knows exactly why, but in the late 1320s or early 1330s, bubonic plague broke out in China’s Gobi desert. Spread by flea-infested rats, it didn’t take long for the disease to reach Europe. In October of 1347, a Genoese ship fleet returning from the Black Sea — a key trade link with China — landed in Messina, Sicily. Most of those on board were already dead, and the ships were ordered out of harbor. But it was too late. The town was soon overcome with pestilence, and from there, the disease quickly spread north along trade routes — through Italy and across the European continent. By the following spring, it had reached as far north as England, and within five years, it had killed 25 million people — one-third of the European population.

The bubonic plague is caused by a bacterium called Yersinia pestis and is characterized by chills, fever, vomiting, diarrhea, and the formation of black boils in the armpits, neck, and groin. Though the disease was originally called the “Great Mortality” and the “Great Pestilence,” the name “Black Death” was eventually adopted because of these black boils, which derive their color from dried blood under the skin caused by internal bleeding. In certain cases the bacterium spreads to the victims’ lungs, causing them to fill with frothy, bloody liquid. This derivation of the disease is called pneumonic plague, and can quickly spread from person to person through the air. It is almost always lethal.

The plague first spread to Britain in 1348, travelling from Bristol to Oxford and London in several days. More than three hundred years later, in 1665, perhaps the worst of the English epidemics broke out in London. That summer, the nobility and clergy fled the city, as some 7,000 people died each week. As many as 100,000 lives were lost before winter killed the fleas and the epidemic tapered off. Contemporary medicine could provide no explanation for the sickness, and most doctors were afraid to offer treatment. In an attempt to keep from being infected, the few physicians who did risk exposure wore leather masks with glass eyes and a long beak filled with herbs and spices that were thought to ward off the illness. Even one person in a household showing plague-like symptoms was enough to mandate a 40-day quarantine for the whole home — a virtual death sentence for everyone living in it.

In September 1665, George Viccars, a tailor in the small, central-England village of Eyam, received a parcel of cloth ridden with plague-infected fleas from London. Four days later, Viccars died. By the end of the month, five more villagers had succumbed to the plague. The panicked town turned to their rector, William Mompesson, who persuaded them to quarantine the entire village to prevent the bacterium from spreading throughout the region. It seemed like suicide. A year later, the first outsiders ventured into Eyam, expecting a ghost town. Yet, miraculously, half the town had survived. How did so many villagers live through the most devastating disease known to man?

Local Eyam lore tells befuddling stories of plague survivors who had close contact with the bacterium but never caught the disease. Elizabeth Hancock buried six children and her husband in a week, but never became ill. The village gravedigger handled hundreds of plague-ravaged corpses, but survived as well. Could these people have somehow been immune to the Black Death?

Dr. Stephen O’Brien of the National Institutes of Health in Washington D.C. suggests they were. His work with HIV and the mutated form of the gene CCR5, called “delta 32,” led him to Eyam. In 1996, research showed that delta 32 prevents HIV from entering human cells and infecting the body. O’Brien thought this principle could be applied to the plague bacteria, which affects the body in a similar manner. To determine whether the Eyam plague survivors may have carried delta 32, O’Brien tested the DNA of their modern-day descendents. What he found out was startling…