1
00:00:06,379 --> 00:00:11,960
Imagine if half the people in your neighborhood, your city, or even your whole country were wiped

2
00:00:11,960 --> 00:00:17,640
out. It might sound like something out of an apocalyptic horror film, but it actually happened

3
00:00:17,640 --> 00:00:23,719
in the 14th century during a disease outbreak known as the Black Death. Spreading from China

4
00:00:23,719 --> 00:00:29,160
through Asia, the Middle East, Africa, and Europe, the devastating epidemic destroyed as much as

5
00:00:29,160 --> 00:00:35,100
one-fifth of the world's population, killing nearly 50% of Europeans in just four years.

6
00:00:35,840 --> 00:00:40,679
One of the most fascinating and puzzling things about the Black Death is that the illness itself

7
00:00:40,679 --> 00:00:46,359
was not a new phenomenon, but one that has affected humans for centuries. DNA analysis

8
00:00:46,359 --> 00:00:51,039
of bone and tooth samples from this period, as well as an earlier epidemic known as the

9
00:00:51,039 --> 00:00:57,000
Plague of Justinian in 541 CE, has revealed that both were caused by Yersinia pestis,

10
00:00:57,000 --> 00:01:00,280
the same bacterium that causes bubonic plague today.

11
00:01:01,960 --> 00:01:05,840
What this means is that the same disease caused by the same pathogen

12
00:01:05,840 --> 00:01:08,739
can behave and spread very differently throughout history.

13
00:01:09,640 --> 00:01:14,260
Even before the use of antibiotics, the deadliest outbreaks in modern times,

14
00:01:14,560 --> 00:01:17,400
such as the ones that occurred in early 20th century India,

15
00:01:17,900 --> 00:01:20,299
killed no more than 3% of the population.

16
00:01:21,159 --> 00:01:25,219
Modern instances of plague also tend to remain localized or travel slowly

17
00:01:25,219 --> 00:01:30,739
as they are spread by rodent fleas. But the medieval Black Death, which spread like wildfire,

18
00:01:31,019 --> 00:01:36,480
was most likely communicated directly from one person to another. And because genetic comparisons

19
00:01:36,480 --> 00:01:41,739
of ancient and modern strains of Yersinia pestis have not revealed any significantly functional

20
00:01:41,739 --> 00:01:47,079
genetic differences, the key to why the earlier outbreak was so much deadlier must lie not in the

21
00:01:47,079 --> 00:01:53,400
parasite but the host. For about 300 years during the High Middle Ages, a warmer climate and

22
00:01:53,400 --> 00:01:58,859
agricultural improvements had led to explosive population growth throughout Europe. But with

23
00:01:58,859 --> 00:02:04,640
so many new mouths to feed, the end of this warm period spelled disaster. High fertility rates

24
00:02:04,640 --> 00:02:09,840
combined with reduced harvests meant the land could no longer support its population, while

25
00:02:09,840 --> 00:02:16,219
the abundant supply of labor kept wages low. As a result, most Europeans in the early 14th century

26
00:02:16,219 --> 00:02:21,800
experienced a steady decline in living standards, marked by famine, poverty, and poor health,

27
00:02:21,800 --> 00:02:27,560
leaving them vulnerable to infection. And indeed, the skeletal remains of Black Death victims found

28
00:02:27,560 --> 00:02:33,639
in London show telltale signs of malnutrition and prior illness. The destruction caused by

29
00:02:33,639 --> 00:02:39,539
the Black Death changed humanity in two important ways. On a societal level, the rapid loss of

30
00:02:39,539 --> 00:02:45,340
population led to important changes in Europe's economic conditions. With more food to go around,

31
00:02:45,580 --> 00:02:49,139
as well as more land and better pay for the surviving farmers and workers,

32
00:02:49,139 --> 00:02:54,219
people began to eat better and live longer, as studies of London cemeteries have shown.

33
00:02:55,060 --> 00:02:58,280
Higher living standards also brought an increase in social mobility,

34
00:02:58,759 --> 00:03:02,139
weakening feudalism, and eventually leading to political reforms.

35
00:03:02,699 --> 00:03:08,659
But the plague also had an important biological impact. The sudden death of so many of the most

36
00:03:08,659 --> 00:03:12,979
frail and vulnerable people left behind a population with a significantly different

37
00:03:12,979 --> 00:03:16,919
gene pool, including genes that may have helped survivors resist the disease.

38
00:03:16,919 --> 00:03:22,740
And because such mutations often confer immunities to multiple pathogens that work in similar ways,

39
00:03:23,159 --> 00:03:28,620
research to discover the genetic consequences of the Black Death has the potential to be hugely beneficial.

40
00:03:29,159 --> 00:03:34,639
Today, the threat of an epidemic on the scale of the Black Death has been largely eliminated thanks to antibiotics.

41
00:03:35,479 --> 00:03:39,900
But the bubonic plague continues to kill a few thousand people worldwide every year.

42
00:03:40,219 --> 00:03:45,020
And the recent emergence of a drug-resistant strain threatens the return of darker times.

43
00:03:45,860 --> 00:03:49,180
Learning more about the causes and effects of the Black Death is important,

44
00:03:49,500 --> 00:03:52,699
not just for understanding how our world has been shaped by the past,

45
00:03:53,039 --> 00:03:56,219
it may also help save us from a similar nightmare in the future.