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Imagine if half the people[br]in your neighborhood, your city,

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or even your whole country were wiped out.

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It might sound like something out of [br]an apocalyptic horror film,

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but it actually happened [br]in the 14th century

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during a disease outbreak[br]known as the Black Death.

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Spreading from China through Asia,[br]the Middle East, Africa and Europe,

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the devastating epidemic destroyed [br]as much as 1/5 of the world's population,

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killing nearly 50% of Europeans[br]in just four years.

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One of the most fascinating[br]and puzzling things abut the Black Death

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is that the illness itself[br]was not a new phenomenon

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but one that has affected[br]humans for centuries.

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DNA analysis of bone[br]and tooth samples from this period,

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as well as an earlier epidemic known as[br]the Plague of Justinian in 541 CE,

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has revealed that both were caused[br]by Yersinia pestis,

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the same bacterium that causes[br]bubonic plague today.

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What this means is that the same disease[br]caused by the same pathogen

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can behave and spread [br]very differently throughout history.

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Even before the use of antibiotics,[br]the deadliest oubreaks in modern times,

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such as the ones that occurred[br]in early 20th century India,

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killed no more than 3% of the population.

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Modern instances of plague[br]also tend to remain localized,

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or travel slowly,[br]as they are spread by rodent fleas.

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But the medieval Black Death,[br]which spread like wildfire,

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was most likely communicated directly[br]from one person to another.

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And because genetic comparisons of ancient[br]to modern strains of Yersinia pestis

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have not revealed any significantly[br]functional genetic differences,

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the key to why the earlier outbreak[br]was so much deadlier

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must lie not in the parasite but the host.

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For about 300 years during[br]the High Middle Ages,

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a warmer climate[br]and agricultural improvements

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had led to explosive population growth[br]throughout Europe.

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But with so many new mouths to feed,

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the end of this warm period spelled disaster.

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High fertility rates[br]combined with reduced harvest,

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meant the land could no longer [br]support its population,

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while the abundant supply[br]of labor kept wages low.

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As a result, most Europeans[br]in the early 14th century

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experienced a steady decline[br]in living standards,

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marked by famine, poverty and poor health,[br]leaving them vulnerable to infection.

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And indeed, the skeletal remains[br]of Black Death victims found in London

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show telltale signs of [br]malnutrition and prior illness.

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The destruction caused by the Black Death[br]changed humanity in two important ways.

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On a societal level,[br]the rapid loss of population

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led to important changes in[br]Europe's economic conditions.

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With more food to go around,

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as well as more land and better pay[br]for the surviving farmers and workers,

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people began to eat better and live longer[br]as studies of London cemeteries have shown.

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Higher living standards also brought[br]an increase in social mobility,

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weakening feudalism,[br]and eventually leading to political reforms.

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But the plague also had an important[br]biological impact.

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The sudden death of so many of[br]the most frail and vulnerable people

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left behind a population with[br]a significantly different gene pool,

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including genes that may have helped[br]survivors resist the disease.

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And because such mutations [br]often confer immunities

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to multiple pathogens [br]that work in similar ways,

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research to discover the genetic[br]consequences of the Black Death

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has the potential to be hugely beneficial.

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Today, the threat of an epidemic [br]on the scale of the Black Death

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has been largely eliminated [br]thanks to antibiotics.

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But the bubonic plague continues to kill a[br]few thousand people worldwide every year,

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and the recent emergence of a [br]drug-resistant strain[br]

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threatens the return of darker times.

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Learning more about the causes and effects[br]of the Black Death is important,

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not just for understanding how[br]our world has been shaped by the past.

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It may also help save us from [br]a similar nightmare in the future.