Your muscles begin to ache,

you grow irritable,

and you lose your appetite.

It's official: you've got the flu.

It's logical to assume that this miserable medley of symptoms

is the result of the infection coursing through your body,

but is that really the case?

What's actually making you feel sick?

What if your body itself was driving this vicious onslaught?

You first get ill when a pathogen like the flu virus gets into your system,

infecting and killing your cells.

But this unwelcome intrusion has another effect:

it alerts your body's immune system to your plight.

As soon as it becomes aware of infection, your body leaps to your defense.

Cells called macrophages charge in as the first line of attack,

searching for and destroying the viruses and infected cells.

Afterwards, the macrophages release protein molecules called cytokines

whose job is to recruit

and organize more virus-busting cells from your immune system.

If this coordinated effort is strong enough,

it'll wipe out the infection before you even notice it.

But that's just your body setting the scene for some real action.

In some cases, viruses spread further,

even into the blood and vital organs.

To avoid this sometimes dangerous fate,

your immune system must launch a stronger attack,

coordinating its activity with the brain.

That's where those unpleasant symptoms come in,

starting with the surging temperature,

aches and pains,

and sleepiness.

So why do we experience this?

When the immune system is under serious attack,

it secretes more cytokines,

which trigger two responses.

First, the vagus nerve, which runs through the body into the brain,

quickly transmits the information to the brain stem,

passing near an important area of pain processing.

Second, cytokines travel through the body to the hypothalamus,

the part of the brain responsible for controlling temperature,

thirst,

hunger,

and sleep,

among other things.

When it receives this message,

the hypothalamus produces another molecule

called prostaglandin E2, which gears it up for war.

The hypothalamus sends signals that instruct your muscles to contract

and causes a rise in body temperature.

It also makes you sleepy,

and you lose your appetite and thirst.

But what's the point of all of these unpleasant symptoms?

Well, we're not yet sure,

but some theorize that they aid in recovery.

The rise in temperature can slow bacteria

and help your immune system destroy pathogens.

Sleep lets your body channel more energy towards fighting infection.

When you stop eating, your liver can take up much of the iron in your blood,

and since iron is essential for bacterial survival,

that effectively starves them.

Your reduced thirst makes you mildly dehydrated,

diminishing transmission through sneezes,

coughs,

vomit,

or diarrhea.

Though it's worth noting that if you don't drink enough water,

that dehydration can become dangerous.

Even the body's aches make you more sensitive,

drawing attention to infected cuts that might be worsening,

or even causing your condition.

In addition to physical symptoms,

sickness can also make you irritable,

sad,

and confused.

That's because cytokines and prostaglandin

can reach even higher structures in your brain,

disrupting the activity of neurotransmitters,

like glutamate,

endorphins,

serotonin,

and dopamine.

This affects areas like the limbic system, which oversees emotions,

and your cerebral cortex, which is involved in reasoning.

So it's actually the body's own immune response

that causes much of the discomfort you feel every time you get ill.

Unfortunately, it doesn't always work perfectly.

Most notably, millions of people worldwide suffer from autoimmune diseases,

in which the immune system treats normal bodily cues as threats,

so the body attacks itself.

But for the majority of the human race,

millions of years of evolution have fine-tuned the immune system

so that it works for, rather than against us.

The symptoms of our illnesses are annoying,

but collectively, they signify an ancient process

that will continue barricading our bodies against the outside world

for centuries to come.