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

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The Greeks fought it by chewing
aromatic resins,

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while the Chinese resorted to egg shells.

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In the ancient Jewish Talmud,

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it's even considered 
legal grounds for divorce.

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

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

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

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like garbage, feces or rotting meat.

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All of these smells come from the activity
of microorganisms,

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particularly bacteria,

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and, as disgusting as it may sound,

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

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Don't panic.

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

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it's actually vital 
for all sorts of things,

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like digestion and disease prevention.

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But like all living things,
bacteria need to eat.

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The bacteria in your mouth 
feed off of mucus,

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food remnants,

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and dead tissue cells.

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In order to absorb nutrients
through their cell membranes,

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

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

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and then break those down even further
into various compounds.

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Some of the foul-smelling 
byproducts of these reactions,

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such as hydrogen sulfide and cadaverine,

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escape into the air

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and waft their way 
towards unsuspecting noses.

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

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may be an evolutionary mechanism
warning us of rotten food

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and the presence of disease.

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Smell is one of our most intimate
and primal senses,

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playing a huge role 
in our attraction to potential mates.

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

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

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which may be why Americans alone

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spend $1 billion a year 
on various breath products.

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Fortunately, most bad breath 
is easily treated.

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The worst smelling byproducts come from
gram-negative bacteria

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that live in the spaces 
between gums and teeth

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and on the back of the tongue.

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By brushing and flossing our teeth,

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using antibacterial mouthwash at bedtime,

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gently cleaning the back of the tongue
with a plastic scraper

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and even just eating a healthy breakfast,

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

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In some cases, 
these measures may not be enough

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

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such as liver disease 
and uncontrolled diabetes.

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Behaviors like smoking 
and excessive alcohol consumption

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also have a very recognizable odor.

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Regardless of cause,

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the bad smell almost 
always originates in the mouth

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and not the stomach 
or elsewhere in the body.

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But one of the biggest challenges lies

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

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and it's unclear why.

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It may be that 
we're too acclimatized

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to the smell inside 
our own mouths to judge it.

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And methods like cupping your hands
over your mouth,

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

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One study showed 
that even when people do this,

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they tend to rate the smell subjectively

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

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But there's one simple,
if socially difficult,

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way of finding out how your breath smells:

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just take a deep breath and ask a friend.