Why do you get a fever when you're sick? - Christian Moro

View full lesson: https://ed.ted.com/lessons/why-do-you-get-a-fever-when-you-re-sick-christian-moro

There are many mysteries around fever, but we do know that all mammals, some birds and even a few invertebrate and plant species feel fever’s heat. It has persisted for over 600 million years of evolution. But it has a significant cost: for every degree increase in temperature, there’s a 12.5% increase in energy required. So, why and how does your body produce a fever? Christian Moro investigates.

Lesson by Christian Moro, directed by Artrake Studio.

In 1917, doctors proposed
an outlandish treatment for syphilis,

the incurable bacterial infection
that had ravaged Europe for centuries.

Step 1: Infect patients suffering
from the later stages of syphilis

with the parasite that causes malaria,

the deadly but curable
mosquito-borne disease.

Step 2: Hope that malarial fevers
clear the syphilis.

And step 3: Administer quinine
to curb the malaria.

If all went according to plan,

their patient would be left alive
and free of both diseases.

This killed some 15% of patients,
but for those who survived,

it seemed to work.

It actually became the standard treatment
for syphilis

until penicillin
was widely used decades later.

And its driving force was fever.

There are many mysteries around fever,

but what we do know is that all mammals,

some birds and even a few invertebrate
and plant species feel fever’s heat.

It has persisted
for over 600 million years of evolution.

But it has a significant cost.

For every 1 degree Celsius of temperature
increase in the human body,

there’s a 12.5 percent increase
in energy required,

the equivalent of about 20 minutes
of jogging for some.

So, why and how does your body
produce a fever?

Your core temperature is maintained
via thermoregulation,

a set of processes that usually keep
you around 37 degrees Celsius.

These mechanisms are controlled
by the brain’s hypothalamus,

which detects minute temperature shifts

and sends signals
throughout the body accordingly.

If you’re too hot, the hypothalamus
produces signals

that activate your sweat glands
or make your blood vessels dilate,

moving blood closer
to the skin’s surface—

all of which releases heat
and cools you off.

And if you’re too cold,

your blood vessels will constrict
and you may start to shiver,

which generates heat.

Your body will disrupt its usual
temperature equilibrium to induce a fever,

which sets in above 38 degrees Celsius.

Meanwhile, it has mechanisms
in place to prevent it

from exceeding 41 degrees Celsius,
when organ damage could occur.

Immune cells that are fighting
an infection can induce a fever

by triggering a biochemical cascade
that ultimately instructs

your hypothalamus to increase
your baseline temperature.

Your body then gets to work to meet
its new “set point” using the mechanisms

it would to generate heat when cold.

Until it reaches this new temperature,
you’ll feel comparatively cool,

which is why you might experience chills.

But why does your body do this?

While the jury's still out on how higher
temperatures directly affect pathogens,

it seems that fever's main effect

is in rapidly inducing a whole-body
immune response.

Upon exposure to raised
internal temperatures,

some of your cells release
heat shock proteins, or HSPs,

a family of molecules produced
in response to stressful conditions.

These proteins aid lymphocytes,
one of several kinds of white blood cells

that fight pathogens, to travel
more rapidly to infection sites.

HSPs do this by enhancing
the “stickiness” of lymphocytes,

enabling them to adhere to and squeeze
through blood vessel walls

so they can reach the areas
where infection is raging.

In the case of viral infections,

HSPs help tell nearby cells to dampen
their protein production,

which limits their ability to replicate.

This stunts the virus’s spread
because they depend on

their host’s replicative machinery
to reproduce.

It also protects surrounding cells
from damage since some viruses spread

by rupturing their host cells, which can
lead to large-scale destruction,

the build-up of detritus,
and potentially even organ damage.

The ability of HSPs to protect host cells
and enhance immune activity

can limit the pathogen’s
path of destruction inside of the body.

But for all we know about fever’s role
in immune activation,

some clinical trials have shown
that fever suppressor drugs

don’t worsen symptoms or recovery rates.

This is why there’s no definitive rule
on whether to suppress a fever

or let it ride.

Doctors decide on a case-by-case basis.

The fever’s duration and intensity,
as well as their patient’s immune status,

comfort level, and age will all play
a role in their choice of treatments.

And if they do let a fever ride,

they’ll likely prescribe rest and plenty
of fluids to prevent dehydration

while the body wages its heated battle.