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Imagine if half the people
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
an apocalyptic horror film,
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but it actually happened
in the 14th century
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during a disease outbreak
known as the Black Death.
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Spreading from China through Asia,
the Middle East, Africa and Europe,
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the devastating epidemic destroyed
as much as 1/5 of the world's population,
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killing nearly 50% of Europeans
in just four years.
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One of the most fascinating
and puzzling things abut the Black Death
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is that the illness itself
was not a new phenomenon
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but one that has affected
humans for centuries.
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DNA analysis of bone
and tooth samples from this period,
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as well as an earlier epidemic known as
the Plague of Justinian in 541 CE,
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has revealed that both were caused
by Yersinia pestis,
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the same bacterium that causes
bubonic plague today.
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What this means is that the same disease
caused by the same pathogen
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can behave and spread
very differently throughout history.
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Even before the use of antibiotics,
the deadliest oubreaks in modern times,
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such as the ones that occurred
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
also tend to remain localized,
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or travel slowly,
as they are spread by rodent fleas.
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But the medieval Black Death,
which spread like wildfire,
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was most likely communicated directly
from one person to another.
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And because genetic comparisons of ancient
to modern strains of Yersinia pestis
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have not revealed any significantly
functional genetic differences,
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the key to why the earlier outbreak
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
the High Middle Ages,
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a warmer climate
and agricultural improvements
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had led to explosive population growth
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
combined with reduced harvest,
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meant the land could no longer
support its population,
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while the abundant supply
of labor kept wages low.
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As a result, most Europeans
in the early 14th century
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experienced a steady decline
in living standards,
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marked by famine, poverty and poor health,
leaving them vulnerable to infection.
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And indeed, the skeletal remains
of Black Death victims found in London
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show telltale signs of
malnutrition and prior illness.
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The destruction caused by the Black Death
changed humanity in two important ways.
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On a societal level,
the rapid loss of population
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led to important changes in
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
for the surviving farmers and workers,
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people began to eat better and live longer
as studies of London cemeteries have shown.
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Higher living standards also brought
an increase in social mobility,
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weakening feudalism,
and eventually leading to political reforms.
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But the plague also had an important
biological impact.
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The sudden death of so many of
the most frail and vulnerable people
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left behind a population with
a significantly different gene pool,
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including genes that may have helped
survivors resist the disease.
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And because such mutations
often confer immunities
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to multiple pathogens
that work in similar ways,
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research to discover the genetic
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
on the scale of the Black Death
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has been largely eliminated
thanks to antibiotics.
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But the bubonic plague continues to kill a
few thousand people worldwide every year,
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and the recent emergence of a
drug-resistant strain
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threatens the return of darker times.
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Learning more about the causes and effects
of the Black Death is important,
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not just for understanding how
our world has been shaped by the past.
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It may also help save us from
a similar nightmare in the future.