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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.