It starts with a tickle in your throat
that becomes a cough. 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.
