Do you like to share?  I hope so.  When you eat a sandwich or some fruit, lots of other living things eat it with you!  Your intestines are filled with millions of very small living things.  They are happy to get food and shelter inside you.  Some of them are good and help you digest.  Others only take, take, take, eating up the best parts of your food while giving you nothing in return.  Let's go inside some intestines and find out what their problem is -- and if we can get those vitamins back.

First of all, why are they in there?  This is your stomach!  Why can't they go find their own food?  Well, this is the only way these small living things can live.  They were made to steal.  A parasite is a living thing that takes its food from other living things without giving anything back.  The victims, or hosts, who have things stolen from them can be anything from humans to plants to dogs to ants!  Some of the most common parasites are fleas or lice.  They hide in an animal's hair and suck its blood.  Now wait, you might feel sorry for them for a second.  Their host is the only way they can live!  That means if their host dies, they die too.  There.  Now that we are done feeling sorry, let's get them out of there!

No wonder fleas can jump. Look at those legs!

This is not going to be easy.  Some parasites are very small, which makes them hard to find.  Think of a stowaway on a giant ship, hiding in a barrel, and stealing food from the ship.  Protozoa are living things that are just one cell and can move around to get their energy from eating things they find.  You can find these little guys almost everywhere -- in dirt, in rivers, in the ocean, even inside you.  A lot of parasites are protozoa, so they are no bigger around than a hair.  Your body seems as big as a country to them!  So how are we going to find them?  Well, we need to know what they eat.

This is the protozoa Giardia feeding on an intestine.

In the sea, protozoa will eat algae, but inside you, they will eat bad bacteria.  Some kinds like the same food you do.  Let's check out a sheep's small intestine.  No, it is not very nice to be in here, but you'd rather find a parasite here than inside you.  When you think of worms, you may think of mud, but there are many kinds that can live inside an animal, too.  Tapeworms are parasites that live inside an animal's digestive system for many years and can grow up to 20 feet long!  Even though they have mouths, they do not eat things with them.  Instead, they soak in food through their skin.  Their "mouth" is used to hook to the inside of the sheep then hold on for dear life while the sheep's intestine tries to push it out.

I don't know about you, but tapeworms scare me.

Think that's gross?  We have not even started yet.  This tapeworm will also lay eggs inside of the sheep.  It does this by cutting off a part of its body, which the sheep poops out.  This part of the worm will then crawl away from the poop onto some fresh grass.  If another sheep is unlucky enough to eat that grass, then the baby tape worm will make a home inside of that sheep.  I hate to say it, but yes, people can get this worm too.  It lives in your intestines and soaks in your food, which leaves you feeling bad even after you have eaten lots of good foods!  You do not need to be too worried, though.  Cooking your meat well and washing your hands after you use the bathroom should kill any tapeworms that might try to make a home in you.

As you now know, you have good things living in you too.  Even though they eat some of your food, they also help break your food down.  Symbiosis is when two living things live together and help each other to survive.  In these cases, two things living together can be good for both.  Think of a human and a dog.  The dog gets food and love.  The human gets a friend and a guard.  However, it's not always so good when two things live together.  We talked about the tapeworm.  Another example are the barnacles that live on a whale.  The barnacles eat.  The whale itches.  It's not a great deal for either one.  Anytime you see two animals living together, think about whether one is getting a better deal than the other.  No, living with your little brother or sister does not count.


How Stuff Works. "Have people ever really eaten tapeworms for weight loss?" How Stuff Works, 2010. <>

Your Dictionary. "Examples of Symbiosis." Your Dictionary, 2011. <>

About Kids Health. "Intestinal Parasite." About Kids Health, 2010. <>

Radiolab. "Parasites." Radiolab, 2010. <>