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Being infected by parasites sounds like something
that happens to other people in faraway places.
However, parasites that infect humans are
far more common than you might think.
A lot of them have no symptoms, either, so
people can go years without knowing they are infected.
Possibly up to 60 percent of the world's population
has at least one parasite in them right now.
That's right, there's better than a 50/50
chance that you have one right now.
Here is a list of ten of the most common asymptomatic
parasites that could be nearer than you might think.
Do you like your steak rare?
Well, tapeworms (Taenia solium or Taenia saginata)
get into human hosts from raw or undercooked meat.
These tape measure shaped parasites can grow
up to 15 meters (50 ft) long.
They like to live in people's intestines and
are transferred from animals, mostly cows and pigs.
This happens either through the consumption
of infected meat or unwashed vegetables.
If a tapeworm larva is eaten, it can grow
into a full-blown adult that feeds off of
the intestinal wall.
This little guy can live in a human gut for
nearly 30 years.
People can grow old with their parasite. How cute.
The tapeworm's eggs, which can develop into
cysts on organs, can be much more dangerous.
If the infection is just the tapeworm
(or worms),
symptoms can be rare or do not appear at all.
It will happily live unnoticed for years.
Liver flukes are parasites that infect the
bile ducts and liver.
They are one of a number of flatworms that
breed in freshwater snails.
One way they can infect humans is through
the consumption of freshwater fish that share
the same environment.
These flat-looking parasites can cause few
symptoms in humans.
Therefore, someone can be infected without
ever knowing it.
Mature flukes will eventually cause chronic
inflammation of the bile ducts, which often
leads to gallstones.
While the majority of cases are found in developing
countries, there have been cases in Hawaii,
California, and Florida.
Two kinds of hookworms that can infect human
hosts are Ancylostoma duodenale and Necator
americanus.
Hookworms make an interesting journey through
the human body.
They enter through the feet and can sometimes
leave itchy rashes at their entry point.
They will travel along the bloodstream until
they can enter the lungs.
Once in the lungs, they irritate lung tissue
enough that the infected host coughs them
up into their mouth.
If even a little bit of that bloody, wormy
phlegm gets swallowed, the hookworm is happily
in its desired home: the small intestine.
This is a parasite that has been particularly
prominent in some communities because it can
be transferred from person to person
through poop.
If infected stool isn't managed properly or
is spread in soil, it can infect others easily.
Therefore, now that we have better sanitation
practices, hookworm infection has diminished,
but it hasn't disappeared.
It has been argued that the American South
was left in an economic lethargy because
of hookworms.
For about 300 years, nearly 40 percent of
the population was infected.
If someone is heavily infected with hookworms,
it can cause lethargy and mental incapacity.
Therefore, with so much of the population
infected, the South didn't have the ability
to keep up with its wormless neighbors.
Otherwise, there are very few symptoms.
Pinworms are a parasite that can spread
very easily.
They are generally the most common parasitic
infection in North America.
They live on the human ***, so it takes just
one scratch, and then anything that person
touches can be covered with pinworm eggs.
If someone swallows an egg, they become infected.
There are generally very few symptoms; sometimes
it's just a mild irritation, which encourages
the infected host to scratch their butt and
keep spreading the eggs.
Mostly, pinworm infections are found in children,
but they can easily infect anyone who has
been exposed to them.
Ascariasis is a roundworm infestation that
attacks the small intestine.
It usually spreads from exposure to human
feces or uncooked meat, however it can also
spread via human-to-human contact.
Usually, it has few symptoms, unless the host
has a large infestation of these roundworms.
Children are the most likely to display symptoms,
and they're often at a higher risk of getting
infected with ascariasis because of their
tendency to put their hands in their mouths.
They can suffer bloated stomachs and pain
as a result.
Mostly though, the little roundworms just
live in your stomach until they are ready
to be pooped out.
Our canine besties can give us a parasite
that will slowly grow inside humans for years
without symptoms.
Echinococcus granulosus mainly infects dogs
but can also get inside humans who have been
exposed to canine feces or have even just
petted a dog.
It is a tapeworm-like larva that will create
cyst-like lesions in the liver or lungs.
Trichinosis is a roundworm infestation that
usually occurs after eating undercooked or
raw meat.
It is mostly found in boar or pig meat.
So if you've been hankering on having a medieval
feast with your chums, make sure to cook that
boar all the way through.
Once the trichinosis larva is in the intestines,
it will develop into a mature worm, which
will produce more larvae that can go through
muscles and other body tissues.
If a small sample of larvae is swallowed,
the infected host might never know that they
have trichinosis.
However, a large infestation causes intestinal
pain and diarrhea.
This single-celled parasite is fairly mysterious.
Not only do scientists and medical professionals
not know how it is transmitted, but they don't
know if it produces any symptoms in
human hosts.
It might cause diarrhea and abdominal pain
in some cases, but mostly, it is asymptomatic.
There have been some connections with pinworms,
and it is conjectured that transmission comes
from eating the eggs, but really we don't
know much about this pervasive parasite.
We do know that Dientamoeba fragilis can infect
large populations without anyone knowing they
have a parasite in them.
This group of single-celled parasitic fungi
can infect a huge range of creatures, and
humans are among them.
Unless the human host is weakened in some
manner (such as being immunocompromised),
the fungi can exist asymptomatically and harmlessly.
These spore-forming parasites often live in fish.
Microsporidia were originally considered to
be protists.
However, their genomes resembled fungi more
than other eukaryotic organisms, and they
are now classified as such.
This parasite might be one of the most famous,
partly because it can be found in nearly a
third of the world's population and partly
because it might alter someone's brain, which
sounds like something out of a science
fiction novel.
Toxoplasma gondii is a parasite aims to be
in the gut of a cat.
How it gets there is fascinating.
It will infect rats and mice that have been
exposed to cat feces.
Then it will travel to the brain and pretty
much take over the rodent's natural fear of
predators, turning it into attraction.
The rodents, instead of running away like
they normally would do, instead seek out cats
and promptly get eaten.
The parasite has then succeeded in its goal
of ending up in a cat's gut.
Where it gets weird is that Toxoplasma gondii
can also infect humans.
There are generally no symptoms unless the
person infected is pregnant.
However, the parasites might also try to change
the brain chemistry of their human hosts,
potentially leading to schizophrenia and suicide.
At least one scientist attributes Toxoplasma
gondii with political dissent and higher rates
of car crashes.
This, however, hasn't been accepted by
mainstream scientists.
But maybe next time you forget to pay your
bills or walk into traffic, you can blame
your cat's parasites.