There is a curse that has plagued humanity
since ancient times.

The Greeks fought it by chewing
aromatic resins,

while the Chinese resorted to egg shells.

In the ancient Jewish Talmud,

it's even considered 
legal grounds for divorce.

This horrible scourge is halitosis,
otherwise known as bad breath.

But what causes it,
and why is it so universally terrifying?

Well, think of some of the worst
odors you can imagine,

like garbage, feces or rotting meat.

All of these smells come from the activity
of microorganisms,

particularly bacteria,

and, as disgusting as it may sound,

similar bacteria live in the moisture-rich
environment of your mouth.

Don't panic.

The presence of bacteria in your body
is not only normal,

it's actually vital 
for all sorts of things,

like digestion and disease prevention.

But like all living things,
bacteria need to eat.

The bacteria in your mouth 
feed off of mucus,

food remnants,

and dead tissue cells.

In order to absorb nutrients
through their cell membranes,

they must break down the organic matter
into much smaller molecules.

For example, they'll break proteins 
into their component amino acids

and then break those down even further
into various compounds.

Some of the foul-smelling 
byproducts of these reactions,

such as hydrogen sulfide and cadaverine,

escape into the air

and waft their way 
towards unsuspecting noses.

Our sensitivity to these odors
and interpretation of them as bad smells

may be an evolutionary mechanism
warning us of rotten food

and the presence of disease.

Smell is one of our most intimate
and primal senses,

playing a huge role 
in our attraction to potential mates.

In one poll, 59% of men and 70% of women

said they wouldn't go on a date 
with someone who has bad breath,

which may be why Americans alone

spend $1 billion a year 
on various breath products.

Fortunately, most bad breath 
is easily treated.

The worst smelling byproducts come from
gram-negative bacteria

that live in the spaces 
between gums and teeth

and on the back of the tongue.

By brushing and flossing our teeth,

using antibacterial mouthwash at bedtime,

gently cleaning the back of the tongue
with a plastic scraper

and even just eating a healthy breakfast,

we can remove many of these bacteria
and their food sources.

In some cases, 
these measures may not be enough

due to dental problems, 
nasal conditions, or rarer ailments,

such as liver disease 
and uncontrolled diabetes.

Behaviors like smoking 
and excessive alcohol consumption

also have a very recognizable odor.

Regardless of cause,

the bad smell almost 
always originates in the mouth

and not the stomach 
or elsewhere in the body.

But one of the biggest challenges lies

in actually determining 
how our breath smells in the first place,

and it's unclear why.

It may be that 
we're too acclimatized

to the smell inside 
our own mouths to judge it.

And methods like cupping your hands
over your mouth,

or licking and smelling your wrist
don't work perfectly either.

One study showed 
that even when people do this,

they tend to rate the smell subjectively

according to how bad 
they thought it was going to be.

But there's one simple,
if socially difficult,

way of finding out how your breath smells:

just take a deep breath and ask a friend.