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It starts with a tickle in your throat
that becomes a cough.
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Your muscles begin to ache,
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you grow irritable,
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and you lose your appetite.
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It's official: you've got the flu.
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It's logical to assume that this
miserable medley of symptoms
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is the result of the infection
coursing through your body,
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but is that really the case?
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What's actually making you feel sick?
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What if your body itself
was driving this vicious onslaught?
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You first get ill when a pathogen
like the flu virus gets into your system,
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infecting and killing your cells.
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But this unwelcome intrusion
has another effect:
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it alerts your body's immune system
to your plight.
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As soon as it becomes aware of infection,
your body leaps to your defense.
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Cells called macrophages charge in
as the first line of attack,
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searching for and destroying the viruses
and infected cells.
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Afterwards, the macrophages release
protein molecules called cytokines
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whose job is to recruit
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and organize more virus-busting cells
from your immune system.
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If this coordinated effort
is strong enough,
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it will wipe out the infection
before you even notice it.
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But that's just your body setting
the scene for some real action.
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In some cases, viruses spread further,
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even into the blood and vital organs.
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To avoid this sometimes dangerous fate,
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your immune system must launch
a stronger attack,
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coordinating its activity with the brain.
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That's where those unpleasant symptoms
come in,
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starting with the surging temperature,
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aches and pains,
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and sleepiness.
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So why do we experience this?
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When the immune system is under
serious attack,
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it secretes more cytokines,
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which trigger two responses.
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First, the vagus nerve, which runs through
the body into the brain,
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quickly transmits the information
to the brain stem,
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passing near an important area
of pain processing.
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Second, cytokines travel through
the body to the hypothalamus,
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the part of the brain responsible
for controlling temperature,
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thirst,
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hunger,
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and sleep,
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among other things.
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When it receives this message,
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the hypothalamus produces
another molecule
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called prostaglandin E2,
which gears it up for war.
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The hypothalamus sends signals
that instruct your muscles to contract
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and causes a rise in body temperature.
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It also makes you sleepy,
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and you lose your appetite and thirst.
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But what's the point of all of these
unpleasant symptoms?
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Well, we're not yet sure,
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but some theorize that they aid
in recovery.
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The rise in temperature can slow bacteria
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and help your immune system
destroy pathogens.
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Sleep lets your body channel
more energy towards fighting infection.
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When you stop eating, your liver can
take up much of the iron in your blood,
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and since iron is essential
for bacterial survival,
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that effectively starves them.
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Your reduced thirst makes
you mildly dehydrated,
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diminishing transmission through sneezes,
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coughs,
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vomit,
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or diarrhea.
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Though it's worth noting that if you don't
drink enough water,
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that dehydration can become dangerous.
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Even the body's aches
make you more sensitive,
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drawing attention to infected cuts
that might be worsening,
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or even causing your condition.
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In addition to physical symptoms,
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sickness can also make you irritable,
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sad,
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and confused.
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That's because cytokines and prostaglandin
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can reach even higher structures
in your brain,
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disrupting the activity
of neurotransmitters,
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like glutamate,
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endorphins,
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serotonin,
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and dopamine.
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This affects areas like the limbic system,
which oversees emotions,
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and your cerebral cortex,
which is involved in reasoning.
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So it's actually the body's own
immune response
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that causes much of the discomfort
you feel everytime you get ill.
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Unfortunately, it doesn't always
work perfectly.
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Most notably, millions of people
worldwide suffer from autoimmune diseases,
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in which the immune system treats
normal bodily cues as threats,
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so the body attacks itself.
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But for the majority of the human race,
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millions of years of evolution
have fine-tuned the immune system
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so that it works for,
rather than against us.
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The symptoms of our illnesses
are annoying,
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but collectively,
they signify an ancient process
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that will continue barricading our bodies
against the outside world
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for centuries to come.