in the year 1900 life in the united
states was very different one in every
125 people died of infectious diseases
every year
it was a tragic fact of life that
pneumonia and the flu regularly killed
people but by 1980 that number was only
about one in every three thousand we
take it for granted that over time
society gets better at tackling
infectious diseases but the fact that
they became more than 20 times less
deadly and just under a century is
pretty astonishing
with diseases robbing fewer people of
long healthy lives life expectancy in
the u.s jumped from 48 to 73 in the same
time span amazingly we see a similar
story in other countries too which
suggests something pretty big is going
on
how did such dramatic changes happen in
such a historically short span of time
one way to approach the question is the
theory of epidemiological transition it
describes how the nature of diseases in
a country changes as the people and
their societies change in this episode
we'll take a look at how this theory
helps us chart the journey's nation's
take to push back infectious diseases
and the challenges they face along the
way i'm party savetti and this is crash
course outbreak science
[Music]
in the 20th century something remarkable
was happening populations were exploding
not literally exploding the size of the
population in many countries was growing
rapidly population size depends on two
key factors there's the rate at which
people are born the fertility rate and
the rate at which people die the
mortality rate if a population remains
at the same size those rates must be
equal to each other so that roughly the
same number of people are born and die
every year but for those countries which
had growing populations their fertility
rate was larger than their mortality
rate and it wasn't that people had
suddenly started having more babies
instead for these countries their
mortality rate had begun to shrink
causing the population to grow and after
some time the fertility rate in those
countries also tended to drop people
stopped having as many kids and
eventually the fertility rate was about
equal to the mortality rate again
keeping the population steady at its new
larger size this process of following
mortality rates later followed by
falling fertility rates is what's known
as demographic transition some countries
underwent this process in the 20th
century or earlier while others are
still experiencing it today the big
question is why as you might know from
history class in the last two centuries
most of the world has undergone some
pretty dramatic social environmental and
economic changes
technology has advanced rates of
literacy have increased and the
production of goods has grown a hundred
fold massively growing economies and
historically demographic transition
happens around the same time as these
other changes in a country suggesting
that those factors are linked to the
changing population on the fertility
side changes to the economic cultural
and social environments in different
countries changed what kind of families
people have and the number of children
from generation to generation
people have lots of different reasons
for deciding whether or not to have kids
but on average they start to have less
children during transition but we're
going to focus on the other side of the
equation mortality rates since that's
the part most affected by disease and
it's also where epidemiology can help us
epidemiologists study how often diseases
occur in different groups of people
we'll take a closer look at their roles
later but for now the important thing is
that since most people tend to die from
some kind of disease epidemiology helps
explain why mortality rates fall during
transitions epidemiologists saw that
before and after these demographic
transitions there are very different
trends in the common causes of death
initially many people in a country seem
to die from infectious diseases like
tuberculosis or dysentery but after a
transition the most frequent cause of
death relates to diseases that aren't
infectious like heart disease and cancer
those second kinds of diseases tend to
be linked to the process of aging and
our lifestyles nowadays they're often
referred to as non-communicable diseases
or ncds since they aren't communicated
between organisms by infectious
pathogens the shift from infectious or
communicable diseases to ncds as a major
cause of death is a vital clue for
understanding why a country's mortality
rate could be decreasing it's also where
epidemiologist abdel omron comes in in a
paper published in 1971 he put forward a
way of understanding this change with
the theory of epidemiological transition
the idea was that the falling mortality
rates during a demographic transition
are due to a smaller fraction of people
dying from infectious diseases relative
to the whole population
that's why there are more deaths from
ncds proportionally since people live
longer and are more likely to die from
an ncd instead the theory also
explicitly stated that the transition
was linked with the broader changes that
these countries went through just like
the theory of demographic transition the
kinds of jobs people have their
surrounding environment the technology
they have access to and the social
changes that come along with them all
impact people's health and what kind of
diseases they get the theory of
epidemiological transition also
recognized that no two countries
transition in exactly the same way in
england and sweden for example the
change was a gradual one over more than
a hundred years while japan saw similar
decline in its mortality over a much
shorter period of time at the time omron
proposed a few distinct models for
transition to explain these differences
today even though the evidence is broad
and complex we have a better idea of how
the change in sorts of diseases that
influence mortality rates comes about
and we now see that epidemiological
transition isn't always straightforward
and there can be bumps along the road we
can see this by considering the
transitions of two made-up countries
jonovia and hankistan although they're
made up the scenarios they face are all
ones that real countries have
experienced let's start with a little
background genovia has a tropical
climate and people there live in big
houses with their extended families
hanging stand meanwhile is more
temperate and lots of its population is
concentrated in a few cities despite
these differences both of them are in a
similar state their populations sadly
have a high mortality rate and half of
all the people end up dying from
infectious diseases hunger or
pregnancy-related health problems
neither one of them is doing so great
economically and people don't live in
the safest and cleanest of environments
but then things start to change
let's go to the thought bubble both
countries experience declines in their
mortality rates but for quite different
reasons hankistan's economy begins to
industrialize and soon on average people
are working more economically productive
jobs and getting a little richer
hankistan invests new money in better
sewer systems and trash collection
taking waste away from where people live
since waste was a major source of
contact with infectious pathogens this
change ensures fewer people get infected
they also put money into health care
systems that allow them to roll out
vaccinations against diseases like
smallpox and enforce sanitation measures
in hospitals like hand washing the
country secures better health treatments
like antibiotics and rehydration salts
now even when people get infected with a
disease they're more likely to survive
all these factors make hankistan's
mortality rate drop way down meanwhile
in johnobia the rats that used to carry
fleas that transmitted plague are going
extinct so by pure luck the country sees
a decrease in its mortality rate
separately over time gianobia's social
landscape changes people move from
houses with their extended families to
smaller individual homes meaning that
infectious diseases can't spread to as
many people in a household hankistan's
investment in public health measures
influence officials in genovia at little
cost local and national governments
start providing better guidance on
quarantining and self-isolating to stop
infection spreading and when jonovi's
economy finally begins to grow too they
invest in public health and adopt
similar measures to hankistan like
vaccination and sanitation but they only
see a small decrease in mortality like i
mentioned these transitions aren't
straightforward and changes that help
one country don't necessarily have the
same effect on another it isn't until
john ovia begins producing and importing
a greater variety of food that their
mortality rates drop to the same level
as hankistan's with more readily
available food hunger and the increased
vulnerability to diseases it creates
kills fewer people thanks thoughtbubble
all in all things have come up for both
jonovia and hankistan but their luck
could change a deadly new virus could
spread from animals to humans leading to
an outbreak in hankistan that devastates
parts of the population their overall
mortality rate would actually rise again
and meanwhile maybe political tensions
in genovia come to a head ending in a
civil war the violence might shed blood
and even destroy some of the existing
infrastructure undoing some of its
benefits mortality would rise there too
eventually hankistan and jonovia could
overcome these challenges the war could
end and preventative measures and
treatments against the virus could take
hold improving both situations so far
we've looked at both hankistan and
johnovia's populations as though they're
just one big group but in the same
country there can be populations who
experience totally different situations
since a lot of its population was there
hankistan's wealth became concentrated
in its big cities which experienced
better health overall compared to poorer
more rural areas of pakistan a new virus
would have a much greater impact outside
of cities since health systems in those
parts of the country wouldn't cope as
well because they lack the equipment and
resources that bigger investment could
have provided many real countries also
have stark health inequalities within
their population
people live in different environments
are exposed to different risks and can
be better or worse off when public
health measures are implemented in
response to an outbreak and of course
it's not like infectious diseases have
disappeared for good after transition
john ovia as we know is a tropical
country and so has a naturally high
number of mosquitoes which is keeping
cases of malaria high
and what's more although fewer people
are hungry thanks to their food policies
changes to lifestyles led to high rates
of obesity which could increase the
likelihood of developing ncds like heart
disease and cancer in fact many
lifestyle factors like whether people
smoke drink exercise or sleep well could
affect how prevalent certain ncds are
and different demographics within
jonovia's population countries that
still face a sizeable amount of
infectious disease and ncds are said to
be facing a double burden of disease
that's particularly challenging because
it requires tackling two related public
health problems together
for example having a chronic condition
like lung disease or cancer can make you
more susceptible to falling seriously
ill or dying when catching an infectious
disease like the flu and even though
countries might not have a high
mortality rate from infectious diseases
all the time as populations get older
the prevalence of ncds becomes high
that means in the case of an outbreak a
country with an older average population
may be more vulnerable than it otherwise
would be so the same social
environmental and economic changes that
drive largely beneficial epidemiological
transitions create new vulnerabilities
as we saw for john ovia and hankistan
the inequalities within and between
countries can even increase the risk of
outbreaks and impact who will be most
affected by them
and that's we'll be looking at in our
next episode we at crash course and our
partners operation outbreak and the
sabeti lab at the broad institute at mit
and harvard want to acknowledge the
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and ongoing relationship with this land
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history of the place you call home
through resources like native
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through the websites and resources they
provide thanks for watching this episode
of crash course outbreak science which
was produced by complexly in partnership
with operation outbreak in the cebety
lab at the broad institute of mit and
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