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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.