In seconds,
its teeth will cut through your skin.
Its suckers will stick into your raw meat,
draining blood from your body.
And there are millions of them!
Could you survive this?
Let's see.
There are about 650 species of leeches.
They can be as small as 15 mm (0.6 in) long
or as large as 45 cm (18 in,)
like the Giant Amazon Leech.
They have a sucker
on both ends of their body.
One of them contains the mouth,
which secretes a strange type of saliva
that numbs its prey.
So, you won't even notice
when they bite into you
and start sucking your blood.
The saliva will also dilate the blood vessels
to increase blood flow,
so they can drink and drink again,
until they quench their thirst for blood.
The average pool holds about
28,000 liters (6,400 gal) of water.
But this time,
the pool is filled with nine million
thirsty leeches.
So, don't make any sudden moves.
Leeches use the disturbances
in the water to find their victims.
Juveniles generally attach to fish
and amphibians.
But adult leeches,
like these ones,
prefer luscious, juicy mammals,
like you.
So, hold your breath
and don't move.
Leeches eat slowly,
so it takes them up to 45 minutes
to quench their thirst.
And they carefully choose
where they will bite you.
They typically target the groin,
between the toes,
armpits,
and hard-to-notice crevices.
And remember,
their numbing saliva
will prevent you from feeling the bite
and realizing that leeches
are draining your blood.
They can even get stuck
inside your mouth and nostrils
and kill you by suffocation.
So, cover your crevices
and don't open your mouth.
A human adult has about
5 liters (1.3 gal) of blood
in their body.
An adult leech drinks
5 to 15 ml (0.84-2.5 tsp)
of blood per meal.
It can take over 300 of these
little bloodsuckers
to suck you dry,
but humans can't live
after losing 40% of their blood.
So it would only take about
120 leeches to kill you.
In 2008,
a 65-year-old man in Turkey
rushed to an emergency room
due to unstoppable bleeding.
Doctors found 130 leech bites
all over his body.
Fortunately, the man survived
after receiving eight units
of frozen plasma.
Would you mind counting
how many leeches have already
attached to your body?
Don't panic and try
to rip them off your body
as they hook into your skin,
which would seriously hurt you.
Common remedies include putting salt,
lemon,
or fire on them to force them off,
but that would make them vomit on you,
and possibly infect you with bacteria.
Instead, find the leech's mouth.
It's on the thinnest part of its body
Use your fingernail, credit card,
or a sharp object
and gently push the mouth
out of your body.
Then wash the wound
and apply alcohol or hydrogen peroxide
to prevent an infection.
If you're swimming,
and water splashes into your mouth
or nose,
leeches can enter your body
and feed on your blood.
In Africa,
people have died from anemia
caused by drinking leech-infested water.
In 2004,
a 15-year-old girl in Tanzania
had severe anemia
and cardiorespiratory distress.
After three days in the hospital,
she vomited,
and a small water leech came out.
Doctors concluded that she drank it
from a leech-infested water source
without even realizing it.
After barfing out the leech,
she began to recover.
So, don't take a single sip
of questionable water.
OK,
you made it out alive,
but you’re not done yet.
You still have to make it through
two more deadly water creatures.
And the next one up is known for
swimming up people’s urethras.
If you don’t want that to happen,
then you better keep watching.
On the waters of the Amazon,
an ancient world flows below your feet.
But make a wrong move,
and part of this world
may end up inside you.
Better get out of the water.
These parasitic catfish,
and their scavenger cousins,
lurk in the basin
of the Amazon River.
The Candiru, the smallest fish
of this family at 2.5 cm (1 in),
feast on the blood of larger fish
by attaching to their gills.
And if you believe the local legends,
they could attach themselves
to a surprising part of your body.
So if you're surrounded
by a school of candiru,
Almost 7,000 km (4,350 miles) long,
the Amazon River is the largest
freshwater system on Earth.
Two-thirds of the world’s species
of animals and plants
depend on this waterway to survive.
But your survival depends
on following these steps.
It's been said that the candiru
hunt by smell.
And the smell that attracts
them the most?
Urine.
Local residents,
and even some websites,
spread the belief that after this fish
smells urine in the water,
it swims toward
and then up the urethra.
Then, the parasite supposedly lives inside
the urinary tract
of this unwilling host.
This claim has long been
contested by researchers,
due to a lack of evidence.
But that doesn't mean
this little creature can't hurt you
in other ways.
This transparent fish
erects short spines
to lodge itself onto hosts.
And after attaching to your skin,
it begins feeding on your blood.
While the inside of your orifices
might be safe from this attack,
your exposed skin
could invite danger.
So make sure to wear swimwear
that fully protects you,
if you want to keep your body
off the menu.
These fish may not prey
on victims through scent,
but they can detect movement.
If you're in a large pool of candiru,
stay still,
and you may stay off their radar.
Try tossing a nearby object
away from you to disrupt the water.
But if this fish does
attach itself to you,
try moving to the shore.
Since it can't live very long
out of the water,
this parasite will die
if it remains on your skin.
People say that
"good things come to those who wait."
If you have the stomach for it,
and don't want to remove a candiru,
you might want to let
this parasite feed.
A candiru will feast
for about 30 to 145 seconds.
When it's had its fill,
this little fish will turn red
from eating your blood,
and swim away.
If you're not the patient type though,
roll up your sleeves.
You have some work to do.
If the fish has made it
into your mouth, nose, or ears,
rinse the area with alcohol.
If it's in a more, uh, visible place,
brace yourself for a bit of pain.
These little spines
will tear at your skin
when you remove them.
Aim for the head, and pull hard.
If you're feeling more rugged,
grab an object with a flat,
sharp edge,
like a small knife or a credit card.
Try sliding the object
under the head to cut off the spines.
And if you're not up for that idea,
don't underestimate
the effect of a good slap.
So let's go over this again.
The urethra isn't large enough
to let the fish get inside you.
Thankfully.
Whew.
A candiru doesn't inject a host body
with any anticoagulants,
so you won't bleed after it lets go.
But whether you successfully
removed the parasite or not,
your wound could become infected.
Treat the wounded area
with alcohol,
and seek medical attention immediately.
No one can deny
the allure of this mysterious corner
of our planet.
But don't take the beauty
of this area for granted.
Hire local guides as escorts
through the river and the jungle.
And make sure
to cover-up properly
while you're swimming,
and prevent any unwanted visitors
from hitching a ride on your body.
But, maybe because candiru
are so small,
you're having a hard time
taking them seriously as a threat.
Fair enough.
So for our last creature,
let’s go with something a little bit bigger
and a whole lot deadlier.
This is the Box jellyfish,
and its venom could kill you in a matter of minutes.
Could you save your life
by peeing on the wound?
Keep watching to find out.
When you hear this music,
you know the first thing
that comes to mind, right?
But, did you know that there's an
even deadlier creature lurking
in the depths?
Something that can kill you
in a matter of minutes,
with excruciating pain?
They're difficult to see,
and their 10-foot long barbed
tentacles contain venom that's
so lethal it's considered to be
one of the deadliest
creatures on Earth.
If you want to survive
this lethal attack,
follow these steps.
OK, swim.
You'll want to get away
as fast as you can.
Box jellyfish are intentional swimmers,
meaning they can propel themselves
in your direction, unlike other kinds
of jellyfish that simply float
along the current.
Scientists have identified more
than 2,000 types of jellyfish,
50 kinds of box jellyfish.
And being stung by one can kill you.
A study of stinging incidents on
a Thai beach found that six
out of 15 were deadly.
If you do get stung, at first it will
feel like you've been bitten,
but there will be not be any evidence
of an attacker.
The sting will have come from the
jellyfish's tentacles, which make jellyfish
Mother Nature's stinging machines.
Each tentacle contains thousands
of stinging cells.
Go deeper, and you'll find nematocysts.
Inside each nematocyst is a stinging
thread, coiled and ready to be launched.
They can fire a microscopic harpoon
at 60 km/h (37 mph).
Inside that harpoon is a spiny, hollow tube
that injects the venom into its victim.
Now, you'll need to get medical
attention right away.
In only three minutes, a box jellyfish sting
can be fatal.
Your heart is about to start racing as your
body freaks out and shuts down.
Muscle spasms, extreme fatigue,
and confusion will set in quickly,
and you'll have difficulty breathing.
Scientist Angel Yanagihara suffered this
horrific sting in 1997, in Hawaii,
while studying for her Ph.D.
Only 500 m (1,640 feet) offshore,
she swam into a swarm of box jellyfish.
It was early, before dawn, and she didn't
see a thing.
But she felt the stinging in
her neck and arms,
and her lungs felt like they were on fire
and starting to collapse.
Miraculously, she made her way
back to shore.
And three days later, after recovering
enough to get out of bed, she decided to
dedicate her life to studying these jellyfish
and help save people from
their fatal venom.
You'll need to work quickly to find
and remove the stingers.
If you're the person doing the removing,
use tweezers, a credit card,
or some other tool.
You don't want to come in contact
with the venom.
Removing the stingers will also prevent
more poison from entering
the person's bloodstream.
Don't use fresh water on the wounds
because that will make
the pain even worse.
Treat the wounded areas with vinegar.
This won't be difficult as the stinging
tentacles will leave thick, rope-like marks
on the victim's body.
The vinegar will help stop the venom from
spreading and provide some pain relief.
And, I don't care what you've read or seen
on the internet,
do not pee on the victim.
This is an urban myth, it does not work,
and it's gross.
The victim has enough
problems right now.
The ammonia and urea, in pee, can help
some stings if they're applied
separately to the wounds.
But our pee contains so much water that
it would likely inflict more pain.
A 10-year-old Australian boy was stung by
a box jellyfish, but under
very fortunate circumstances.
He was in the middle of a swimming
lesson and surrounded by lifeguards.
He was about 50 m (164 ft) offshore
when he was stung.
Immediately, he was brought back to the
beach and given oxygen as other
lifeguards poured vinegar on his wounds.
He was taken to a nearby hospital with an
elevated heart rate but stabilized
shortly after that.
After an ECG and some ibuprofen,
he had a hot shower and was released
within a few hours.
Knowing the dangers of these creatures,
where they live, and how they behave will
go a long way in preventing
this from happening.
Box jellyfish can be found a few hundred
meters (or feet) offshore.
They live in all oceans, some freshwater
lakes, and ponds.
The most lethal species are found
in northern Australia.
Scientists consider them to be the most
venomous marine animal.
They have an incredible range in size,
from the size of a thimble to a diameter
of 2.4 m (8 ft).
Their tentacles can stretch for more than
60 m (200 ft) in length.
Box jellyfish are known to kill, on average,
one person each year in Australia, and up
to six in the Philippines.
But, if you stay away from warmer water,
where these killers hang out,
they're not something you'd necessarily
have to worry about.
So, do you still think that sharks
are the deadliest predator?
These jellyfish are nearly invisible
in the water, don't leave blood behind
when they attack, and they can kill you
much faster than Jaws ever could.
So maybe the best way to survive
all these deadly marine creatures is just to
stay out of the water in the first place.
But it’s not like it’s any safer on land,
that’s where you’ll find some of the
deadliest creatures on Earth.
Want to know which one kills
100,000 people every year?
Find out here on How to Survive.