How pandemics spread

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We live in an interconnected, an increasingly globalized world. Thanks to international jet travel, people and the diseases they carry can be in any city on the planet in a matter of hours. And once a virus touches down, sometimes all it takes is one sneeze to spread the infection throughout the community.

When humans were hunter-gatherers, roaming the wild savannas, we were never in one place long enough, and settlements were not large enough to sustain the transmission of infectious microbes. But with the advent of the agricultural revolution 10,000 years ago, and the arrival of permanent settlements in the Middle East, people began living side-by-side with animals, facilitating the spread of bacteria and viruses between cattle and humans.

Epidemics and pandemics come in many shapes and forms. In 2010, for instance, a devastating earthquake struck Haiti, forcing thousands of people into temporary refugee camps. Within weeks, the camps had become breeding grounds for cholera, a bacteria spread by contaminated water, triggering a country-wide epidemic. But the most common cause of epidemics are viruses, such as measles, influenza, and HIV. And when they go global, we call them pandemics.

Pandemics have occurred throughout human history. Some have left scars on the tissue and bone of their victims, while evidence for others comes from preserved DNA. For instance, scientists have recovered DNA from the bacteria that transmits tuberculosis from the remains of ancient Egyptian mummies. And in 2011, scientists investigating a plague pit in the city of London were able to reconstruct the genome of Yersinia pestis, the bacterium responsible for the Black Death of the 14th century.

It is thought the plague originated in China in around 1340, spreading west along the Silk Road, the caravan route running from Mongolia to the Crimea. In 1347, the plague reached the Mediterranean, and by 1400, it had killed in excess of 34 million Europeans, earning it the title, the Great Mortality. It was later historians who called it the Black Death.

However, by far the greatest pandemic killer is influenza. Flu is constantly circulating between the Southern and Northern Hemispheres. In North America and Europe, seasonal flus occur every autumn and winter. As the majority of children and adults will have been exposed to the virus in previous seasons, these illnesses are usually mild. However, every 20 to 40 years or so the virus undergoes a dramatic mutation. Usually, this occurs when a wild flu virus circulating in ducks and farm poultry meets a pig virus, and they exchange genes.

This process is known as antigenic shift and has occurred throughout human history. The first recorded pandemic occurred in 1580. The 18th and 19th centuries saw at least six further pandemics. In terms of mortality, none can compare with the Great Flu Pandemic of 1918. The first indication of the pandemic came in the spring when American troops in northern France began complaining of chills, headaches, and fever.

Then, the following September, at a U.S. Army barracks near Boston, soldiers started collapsing on parade, prompting their removal to the camp infirmary. As a surgeon there recalled, two hours after admission, they had the mahogany spots over the cheekbones, and a few hours later, you can begin to see the cyanosis extending from their ears and spreading all over the face. It is only a matter of a few hours then until death comes, and it is simply a struggle for air until they suffocate.

On the S.S. Leviathan, a huge American transport en route to Bordeaux, sick men hemorrhaged blood from their noses, turning the decks between their bunks slick with bodily fluids. Meanwhile, British soldiers returning from northern France on furlough introduced the flu to Dover and other Channel ports, from where the virus was carried by rail to London.

By the time the pandemic had run its course in April 1919, an estimated 675,000 Americans and 230,000 Britons were dead. In India alone, some 10 million were killed, and worldwide the death toll was an astonishing 50 million. But that was then. Today, planes can transport viruses to any country on the globe in a fraction of the time it took in 1918.

In February 2003, for instance, a Chinese doctor arrived at the Metropole Hotel in Hong Kong feeling unwell. Unknown to him, he was harboring a new animal-origin virus called SARS, short for Severe Acute Respiratory Syndrome. Within 24 hours of checking into Room 913, sixteen other guests had been infected, and over the following days five boarded planes to overseas destinations, spreading the virus to Vietnam, Singapore, and Canada.

Flights between Hong Kong, Toronto, and other international cities were quickly grounded, and thanks to other emergency measures, a pandemic was averted. By the time the outbreak was over four months later, SARS had infected 29 countries worldwide and more than 1,000 people were dead.

For all that the virus was rapidly contained, however, there was little that could be done about the alarming news reports carried by cable news channels and the Internet. As bloggers added to the hysteria by spreading unfounded conspiracy theories, tourism in Hong Kong and other affected cities ground to a halt, costing businesses more than 10 billion U.S. dollars.

One business, however, did very well. Above all, SARS was a reminder that pandemics have always been associated with panic. If history teaches us anything, it's that while pandemics may start small, their impacts can be as dramatic as wars and natural disasters. The difference today is that science gives us the ability to detect pandemics right at the very beginning and to take action to mitigate their impacts before they spread too widely.

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