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infectious diseases have had some pretty
major impacts on human history and
that's putting it mildly take the black
death of the Middle Ages which wiped out
more than a third of the population of
Europe or smallpox which hitched a ride
to the Americas on ships and decimated
native peoples we've been haunted by
microbial enemies but sometimes our
drive to understand them has laid the
foundations of modern science so here
are six of history's most devastating
diseases and how they affected us in
some pretty big ways we'll start our
list with plague caused by the bacteria
your sania pestis it's transmitted to
people when they're bitten by fleas
carried by rodents most famous for the
14th century outbreak called the Black
Death plague bacteria actually caused
three different forms of the disease
you've probably heard of the most common
one bubonic plague it's when these
bacteria target the lymphatic system
which helps protect your body from junk
like toxins between two and six days
later infected victims get a high fever
headaches and vomiting plus they get
swollen lymph nodes called buboes which
give the plague its name bubonic plague
causes plenty of damage on its own but
if it's left untreated it can develop
into another form though these kinds can
also occur on their own if the bacteria
infect the bloodstream it's called
septicemia plague they can cause clots
that keep blood from reaching tissues
which turn black as they die or if the
microbes infect the lungs it's called
pneumonic plague this can lead to bloody
coughs and rapid death and lets people
transmit the plague through tiny
droplets in the air today all forms of
plague can be treated using antibiotics
but they used to kill half or more of
infected patients which had some huge
effects on society the first confirmed
plague epidemic was called the Justinian
plague it swept through the Roman Empire
starting in 541 C II and contributed to
its fall in the Roman capital of
Constantinople
it's estimated that upwards of 5,000
people per day were killed at its peak
the second big wave reached Europe in
1347 when merchant ships arrived in
Italy full of six sailors the black
death swept through the continent in
just a few years and during that time
it's estimated that a quarter to half of
all Europeans died some 25 million
people over the next couple centuries
plague kept cropping up and the first
quarantines were implemented at the time
it was thought that infectious diseases
were transmitted by bad air so officials
tried to isolate sick people and those
trav
from places that had an outbreak to
prevent more deaths and even though the
science was wrong quarantines helped
prevent the spread of pneumonic plague
between people and control the rats with
fleas that carried plague bacteria the
last huge wave of outbreaks began in
1894 in rural China sweeping through
Asia and Australia and finally after a
few decades scientists discovered the
bacteria and carriers behind it all
which let us start squashing out this
disease smallpox was also a major cause
of death in the past and killed nearly
30 percent of all people who had it it's
a disease caused by the variola virus
which starts with high fever and
headache then small bumps full of
infectious fluid appear all over those
are the pox and in survivors they
eventually scab and turn into scars
humans infected each other through tiny
droplets coughed or sneezed through the
air but in a super gross twist the pox
fluids and crusty scabs got all over
clothes and blankets and could infect
new hosts too now smallpox was
devastating for lots of human history
but it hit especially hard when
colonists from Europe invaded the
Americas the native populations had
immune systems that were adapted to
fight off local diseases not foreign
ones so the variola virus infected and
killed huge numbers of them overall
diseases like smallpox may have caused
the death of up to 90% of the Native
American population smallpox likely
helped the Spanish conquer the Aztec
empire too though it's not nearly on the
same scale the European colonists were
also hurt by the disease like some
estimates think that George Washington
lost more troops to the smallpox
epidemic of 1775 to 1783 the
Revolutionary War casualty rates got a
little better after an army wide
variolation or intentionally infecting
people with a bit of gunk from a patient
scabs to hopefully help them build
immunity exposing people to the virus in
a controlled way ended up being safer
than normal but they still suffer from
some symptoms and there was a risk of
death later doctors tried to make it
less dangerous by using people or
animals infected with cow pox a less
harmful cousin of smallpox caused by the
vaccinia virus this process was called
vaccination and led to the eradication
of smallpox and to all the vaccines we
rely on today speaking of diseases that
crossed oceans let's talk about syphilis
scientists think this sexually
transmitted bacterial infection may have
made its way to Europe in colonists
returning from the Americas it's caused
by Treponema pallidum bacteria and the
initial symptom
aren't fun rashes sores fever headaches
and muscle pain it's transmitted by
direct contact with the sores or passed
down from an infected mother to her
child after a few weeks or months though
the rashes and sores disappear and the
disease goes into a latent stage where
it can be detected in a blood test but
it doesn't cause any symptoms and in up
to a third of untreated cases the
disease comes roaring back to cause
dementia dysfunction of multiple organs
lots of pain and death the first
recorded outbreak of syphilis began in
1495 after a victory celebration by the
French army with infected sex workers so
the people of the time started to call
it the French disease and it was pretty
deadly possibly because the disease was
new to Europe and people didn't have any
immune resistance to it it's hard to say
how many people syphilis killed because
we didn't have any medical records that
tracked cause of death not to mention
sexually transmitted infections were
considered shameful so many people tried
to hide them or pass them off as other
things like leprosy what we do know is
that the disease ravaged the world until
one of the first antibiotics ever
developed to put an end to it around the
turn of the 20th century the
immunologist dr. Paul Ehrlich had
discovered that certain dyes only bonded
to specific types of cells in his lab
his finding led him to believe that
certain compounds could target
disease-causing agents like bacteria
without attacking healthy tissues a
treatment that would later be called
chemotherapy using a systematic
screening process Ehrlich found a
chemical that he developed into an anti
syphilitic drug called Solverson it
quickly became the most prescribed drug
in the world and the process that led to
its discovery earned Ehrlich the Nobel
Prize in Physiology or medicine in 1908
caused by a bacterium called Vibrio
cholerae cholera is a severe
gastrointestinal disease that causes
vomiting and diarrhea
as soon as 12 hours after infection
these bacteria produce toxins that bind
to small intestine enzymes that control
water secretion from the rest of the
body specifically the toxin makes these
enzymes flood the intestines with water
and that leads to dehydration so severe
that it's deadly there were reports of
similar sounding diseases in India as
early as 1000 CE II but cholera didn't
become a global problem until the 19th
century when widespread trade started
happening
cholera caused a lot of fear wherever it
went and even today it's a public health
problem with estimated millions of cases
a year and a hundred thousand deaths
nowadays we know that cholera is spread
through drinking water that's been
contaminated by
infected poop particles so when it hit
England in the 1830s medicine was ruled
by the idea that disease whether it's
black plague or cholera was caused by
bad air from corpses impure people or
even from swamps so in 1854 when a
doctor named John Snow traced almost
every victim of a cholera outbreak in
London to a single water pump nobody
really believed him town officials
removed the pumps handle which kept
people from drinking that water to
humour him and new cases of cholera
dropped off sharply but people still
didn't buy his ideas until a local
Minister set out to prove him wrong and
that failed spectacularly his report
actually ended up tracing the outbreak
to a dirty diaper from a baby who'd
contracted cholera outside of London dr.
snow's revolutionary methods to track
infection patterns and find the source
of an outbreak is why he's considered
one of the fathers of modern
epidemiology so it turns out this John
Snow might have known something after
all mosquitoes are a well-known pest
when it comes to spreading disease and
yellow fever is no exception it's a
viral infection and most people infected
with flavivirus experience symptoms like
fever chills aches fatigue and vomiting
an unlucky 15% or so of patients have it
much worse with bleeding jaundice and
multi organ failure which can lead to
death today we have a vaccine for yellow
fever but that wasn't the case in the
1880s when the French started building
the Panama Canal
we knew that yellow fever was a thing
but not how it was transmitted so we
couldn't stop people from getting sick
more than 20,000 workers died of either
yellow fever or malaria or a fun
combination of both so the French quit
construction in 1889 it wasn't until the
1900s that we discovered mosquitoes were
the culprit and the US was able to fight
the disease and finish the canal they
drained pools of water near towns and
houses which is where mosquitoes lay
their eggs and they covered water that
they couldn't drain with films of oil to
smother the larvae that had already
hatched and they dumped pesticides
everywhere else trying to kill all the
mosquitoes they could find the last
disease in our list is a bit different
from the others it's not transmitted
it's inherited and that's why it used to
be called the Royal disease hemophilia
is a disorder that makes it hard to form
blood clots causing victims to bleed out
from minor wounds that would normally
seal up humans have 20 different
proteins that help form blood clots but
hemophilia is caused by problems in just
two and both of the genes involved are
on the X
chromosome humans have 22 pairs of
non-sex chromosomes so you have two
copies of each gene plus a 23rd pair
that's usually either XX or XY and you
get half of your genes from each parent
for a dominant trait you just need one
of your two genes to express the trait
like having a widow's peak recessive
traits on the other hand need two copies
to be expressed so people can have only
one copy not express the trait and still
pass it on to their kids
they're called carriers now recessive
traits on sex chromosomes can work a
little differently because they're not
necessarily identical people who are XY
don't have a backup copy of either set
of genes so recessive diseases caused by
genes on the X chromosome just get
expressed and that's exactly what
happened with hemophilia historically to
try and remain pure Bloods the royal
families of Europe were notorious for
incest and two related people have a
higher chance of both being carriers for
the same recessive trait because they
share more genes than unrelated people
Queen Victoria of England was a carrier
for haemophilia and one son and three
grandsons bled out from minor injuries
by early adulthood the most famous of
her dangerously bleeding descendants was
the russian prince alexei romanov the
Romanov family kept their only heirs
sickness a secret not wanting to appear
weak and they put their trust in the so
called magical healing abilities of
Siberian grigori rasputin the perceived
influence Rasputin had over the royal
family led to tension with the general
public and may have hastened the
execution of the royal family in 1918
during the Russian Revolution
so maybe recessive disorders are why the
Lannisters and Targaryen czar so messed
up the perceived influence Rasputin had
over the royal family led to tension
with the general public and may have
hastened to the execution of the royal
family in 1918 during the Russian
Revolution so history is shaped by lots
of forces especially things like human
health these six diseases caused
widespread death change societies and
revolutionized what we know about
medicine in ways that still impact us
today thanks for watching this episode
of scishow which is produced by
complexly a group of people who believe
the more we learn the better we get it
being humans if you want to learn more
about human health that check out one of
our sister channels healthcare triage at
youtube.com slash healthcare triage
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