your body is a fortress crafted through
millions of years of evolution and i
don't mean just your fists or your teeth
the little things like your skin tear
ducts and even the hairs in your
nostrils are all designed to defend you
that's because they're protecting you
from even tinier things pathogens the
microscopic organisms that make us sick
you might have heard of them in vague
terms like bugs or germs but the small
world of pathogens is actually
incredibly diverse sometimes weird and
often pretty dangerous and in this
episode we're going to get to know that
world really well from the little
creatures that live there to how our
bodies protect us from the ones that
could make us sick i'm party sabetti and
this is crash course outbreak science
[Music]
in our last episode we saw how looking
at infectious diseases from a
microbiology perspective can help us
understand and tackle outbreaks better
to a microbiologist the roots of disease
are infectious agents the microbes in
large molecules that are transmitted
between larger organisms like humans to
be more specific it all starts with
pathogens which is what we call the
specific infectious agents that can make
us sick pathogens tend to be microbes
like bacteria viruses protozoa and fungi
there's a huge variety of pathogens out
there we need a whole other series to
describe them all but in general they
all have a few key features that help
biologists tell them apart let's find
out who's who first up are viruses which
are made up of fragments of genetic
material wrapped in a kind of coat made
of proteins unlike most other pathogens
they don't have cells and depending on
who you talk to they may not even
qualify as living things that's because
they need to infect a cell and use its
resources to reproduce
i'm team living thing myself they do
this by latching onto a host cell
injecting their own genetic material
into it and taking over the cell's
functions to multiply if they attack
enough cells viruses disrupt the
workings of our organs causing us to get
sick smallpox the common cold flu ebola
polio and cova 19 are all caused by
viruses wow that's a lot of diseases
that's part of why we often focus on
viruses and outbreak science our next
microbe bacteria do have cells they're
single-celled organisms but while other
kinds of cells keep their genetic
material inside a nucleus a bacteria's
genetic material is wrapped up in
circular loops that float freely inside
them not all bacteria are bad there's
friendly bacteria like the ones in our
stomachs that help us digest food and
the ones we use to create fermented
foods like kimchi and yogurt but the
pathogenic kind are much nastier once
inside the body they can kill yourself
through direct attacks or by creating
toxins that paralyze them
other kinds of bacteria multiply so
rapidly they damage entire organs that's
what the bacteria that cause diseases
like cholera and tuberculosis do
protozoa the next microbes on our list
are a little more like us their
single-celled organisms yes
but they're eukaryotes which means they
have a nucleus like our cells do and
they're undoubtedly alive when they get
into our bodies they can harm us in ways
similar to how bacteria can one of the
most widespread infectious diseases
malaria is caused by protozoa carried by
mosquitoes then there are fungi these
are your molds yeasts mushrooms and
they're also eukaryotes some are made up
of single cells some are multicellular
some are harmless pizza toppings and
some make us sick fungi release tiny
cells that can reproduce on their own
called spores
certain kinds of pathogenic spores
travel easily in the air where they can
stick to our skin or be inhaled
during an infection fungal cells
multiply growing into places they
shouldn't and feasting on the cells they
infect fungi are responsible for certain
skin diseases such as athlete's foot and
ringworm and other unpleasant things
like oral thrush finally there are few
pathogen oddballs like parasitic worms
unlike the others they're animals
animals that live inside people by
feeding off what they eat they can even
grow large enough to be seen by the
naked eye i won't mince words here it's
pretty gross so let's move on to the
equally weird and fascinating prions
ions are just proteins that have ended
up folded in the wrong shape it doesn't
sound like a bent out of shape protein
would do much harm but they can be
seriously dangerous if they come into
contact with other correctly folded
proteins inside the body those proteins
become misshapen too those newly
misshapen proteins bend other proteins
out of shape and so on damaging the
organ they're part of that's why we
consider prion diseases infectious
diseases prions can be inherited or
consumed in certain kinds of food one
example is crochet yacob disease or mad
cow disease which occurs in the brain
okay that sounds terrifying but luckily
prions are super rare microbiology is a
pretty large field and we've skimmed a
lot of the details but it should give
you an idea of the many kinds of
pathogens that might enter the body the
question is how do they do it on close
inspection the human body has a lot of
holes as i mentioned in our last episode
science demands clarity so there's no
shying away from the details here some
of the holes in the body are obvious
like your mouth and your nostrils others
aren't as apparent like your tear ducts
ears anus or genitals and although your
skin is quite a good barrier against
pathogens tiny scratches wounds or bites
can create holes too all of these holes
are the roots pathogens can take to get
inside you for example pathogens can be
transmitted by direct contact with an
infected person's skin or bodily fluids
which is often the case for sexually
transmitted infections they can also be
picked up from the surface as we touch
with our hands and enter our bodies when
we later touch our eyes our mouth or our
nose or an infected person might release
droplets containing pathogens when they
talk cough or sneeze which then get
inhaled by someone else it could even be
more straightforward some pathogens find
their way into our food and water which
we unknowingly put straight into our
mouths others like malaria are carried
by animals known as vectors vectors are
typically bloodsucking arthropods like
mosquitoes ticks and fleas and one bites
us to feed they're basically creating
another hole through which they transmit
pathogens directly into our bloodstream
it seems like the drawbridge is wide
open for invaders as far as the human
body is concerned hardly the most
well-protected fortress but your body
has a whole host of features to defend
you from pathogens together these
features form the immune system it all
starts with physical barriers which
prevent pathogens from entering in the
first place skin physically stops
pathogens from getting into our bodies
what's more it's slightly acidic which
prevents bacteria from growing on it and
our sweat contains enzymes that break
down bacterial cell walls our eyes are
similar our eyelashes and eyelids
physically prevent airborne pathogens
from reaching our eyes while our tears
contain antimicrobial compounds that
kill anything our eyelashes miss other
potential entry holes into our body like
our nostrils lips ears genitals and anus
are lined with mucus which physically
traps pathogens
stopping them from getting any further
into you and though it might spread
disease if we're already sick coughing
and sneezing can eject unwanted material
from our airways that contain pathogens
finally we can eject microbes out the
other end every time we use the bathroom
we're also flushing out lots of unwanted
microbes from our systems these physical
barriers are like the walls turrets and
moats of the fortress providing a first
line of defense but should any stubborn
pathogen manage to break through the
second line of defense kicks in the
innate immune system
this system has dedicated cells that
attacked any trespasser so we'll call it
a non-specific barrier monocytes cruise
along your bloodstream looking for
anything suspicious while macrophages
and dendritic cells keep an eye on your
tissues if they find something they can
digest the intruder and macrophages will
eat anything dangerous looking even
tattoo ink in your skin when a
macrophage begins its fight it calls for
help by releasing proteins called
cytokines as a distress signal
at that point tougher cells like
neutrophils and natural killer cells yes
that's their real name will swoop in to
help destroy tougher threats so the
cell's innate immune system are like the
guards of the fortress well trained to
neutralize most enemies that make it
beyond the physical barriers of your
body but occasionally the body needs a
more specific approach in tackling a
pathogen and calls for special forces
that's where the adaptive immune system
comes in unlike the innate immune system
the adaptive immune system is highly
specific its cells target distinct
pathogens and continually well adapt to
be stronger the next time two important
members of the specialized team are the
b cells and t cells b cells are a type
of white blood cell that creates
antibodies which are special custom-made
proteins designed to stick onto one
specific pathogen if an antibody binds
the pathogen is looking for it the body
triggers an immediate immune response to
rapidly destroy the threat that can look
like blocking pathogens from getting
into our healthy cells or making
pathogens clump together stopping them
from infecting more cells and making
them easier for other immune cells to
eat t cells also look for specific
pathogens but they do it a little
differently while b cells in their
antibodies seek out pathogens directly t
cells recognize our own infected cells
when they find one they call them
reinforcements cytotoxic t cells and
helper t cells cytotoxic t cells are in
charge of destroying the infected cells
while helper t cells coordinate the rest
of the response they help b cells
produce antibodies by nudging them into
action or releasing cytokines the
protein distress signals we talked about
earlier our adaptive immune system has a
secret weapon that gives us an advantage
against repeated infections from the
same pathogen it remembers pathogens
it's seen before so can recognize them
more quickly the next time when t cells
or b cells are exposed to pieces of a
digested pathogen they can specialize
into memory cells this process is called
immunological memory memory t cells are
like historians documenting the
invader's attack and storing that data
in our body's long-term memory memory b
cells meanwhile hang out in the body
after the first immune response ready to
spot the pathogen and make antibodies
quickly if it shows up again the
adaptive immune system is like an elite
guard of soldiers and military
intelligence that strategizes to defeat
the more serious threats to your body
and it's this system that we take
advantage of when we make vaccines they
help our t and b cells recognize a
particular pathogen and prep to defend
our bodies against it without making us
seriously sick we'll be talking about
vaccines in more detail in future
episodes unfortunately even with all
these remarkable layers of protection
sometimes things can still go wrong
pathogens are often sneaky and have
multiple ways of evading even our
strongest defenses sometimes we do get
sick or even get sick multiple times
from the same virus our immunity also
varies from person to person so what
makes one person too sick to get out of
bed might look like it doesn't affect
the next person at all our immune system
can even overreact to something that
isn't actually a threat like a particle
of pollen in that case the body will
start up the immune response releasing
the same cytokine distress signals that
normally would which can cause
inflammation and swelling you might
already know this process by another
name allergies in the case of hay fever
it may just be annoying but a serious
food allergy for example could cause
anaphylaxis when the throat swells up so
much that you can suffocate similarly an
autoimmune disorder like multiple
sclerosis is when a body is essentially
allergic to itself and the immune system
attacks our own healthy cells on the
whole though the immune system does a
remarkable job fending off the many
kinds of pathogens it faces
understanding these threats and
supporting the immune system is a
crucial part of tackling diseases during
an outbreak individual bodies are just
one part of the picture in our next
episode we'll be zooming out to look at
how when groups of people change the way
diseases affect them changes too we at
crash course and our partners operation
outbreak and the sabetti lab at the
broad institute at mit and harvard want
to acknowledge the indigenous people
native to the land we live and work on
and their traditional and ongoing
relationship with this land we encourage
you to learn about the history of the
place you call home through resources
like native land dot ca and by engaging
with your local indigenous and
aboriginal nations 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 sabeti lab at the broad
institute of mit and harvard with
generous support from the gordon and
betty moore foundation if you want to
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