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Olivia: What do you think of when you hear the word parasite? Maybe blood-sucking leeches or swarms of mosquitoes spring to mind. Or maybe your dog had a tapeworm one time. A parasite is any living thing that lives and gets its food at the expense of another, generally on or in its host.

And some of them cause some nasty diseases. But some parasites take that in thing to extremes. They don’t just live attached to another organism, or hide out in its gut.

No, they live inside their host’s very cells. Some of them are huge problems for humans, while others are mere curiosities. And while some are bacteria, or other tiny organisms, others aren’t what you’d expect at all.

Here are 6 such tiny parasites, and how they do it.

First up on our list is a group of microbes belonging to the genus Plasmodium, parasites that cause malaria in humans. As you may have heard, this parasite kills a lot of people. Plasmodium has a complicated life cycle that takes it from a mosquito’s gut to its saliva to a human liver to the bloodstream. And that last part is what causes malaria symptoms.

After multiplying in the liver for a while, the parasite bursts out and attacks red blood cells. Incidentally, a single liver cell can produce thousands of Plasmodium cells, and a red blood cell can breed a few dozen. Once it’s inside a red blood cell -- and this is what makes malaria so tricky to treat -- Plasmodium starts rearranging the furniture.

This includes sticking new proteins on the surface of the cell, and creating tiny knobs that cause infected red blood cells to stick to blood vessel walls. That prevents them from flowing through the bloodstream and getting thrown out by the spleen, which normally identifies defective blood cells. The infected blood cell can also clump together with uninfected red blood cells, disguising itself among the healthy cells in something called a rosette.

And those clumps can block traffic.

If these sticky, nonfunctioning red blood cells bunch up in the wrong spot in a blood vessel, they can deprive that area of oxygen. And you definitely don’t want that to happen in your brain or other vital organs.

But the ability to make those little proteins that stick out of the cell is also what makes malaria so hard to detect by our immune systems. Plasmodium has about 60 genes that can code for a variety of these proteins. And research published in 2017 reported that different patterns of those genes show up in different patients, and that likely hinders our ability to develop immunity.

There is a ton of research ongoing, but malaria still kills hundreds of thousands of people every year, mainly in Africa. So there is still a lot of work to do.

Number two on our list is Rickettsia, a group of tiny bacterial parasites transmitted to humans by creepy crawlies like fleas and ticks. And you’ve likely heard about the diseases these parasites cause — sicknesses like Rocky Mountain Spotted Fever or typhus. These are some of the nastier, more deadly parasitic infections out there. The U.

S. Centers for Disease Control have even listed them as potential tools for bioterrorism. Now, if a tick with Rickettsia bites you, the bacteria get into your bloodstream.

But unlike Plasmodium, they don’t target red blood cells. Rather, Rickettsia tends to live in the cells lining your blood vessels. The infection causes various problems with the circulatory system and widespread inflammation.

And if it gets severe enough, the patient can face pneumonia, swelling in the lungs, or kidney failure. All life-threatening stuff. Researchers are currently trying to understand how these bacteria can get into our cells so easily.

And of course, how to prevent them from doing so. A 2015 paper found that proteins on the surface of Rickettsia cells can actually induce host cells to grab them and pull them in. Such entry mechanisms aren’t exclusive to Rickettsia, but we’re interested in learning about them so we can use that in preventing these dangerous diseases.

We can treat Rickettsia infections with antibiotics, but prevention is always better.

So be careful out there if you’re hiking through tick country.

Another one of those human-infecting parasites is Legionella pneumophila, the bacterium behind Legionnaires’ disease and Pontiac fever. As the name pneumophila implies, this type of bacteria tends to attack the lungs. And it can cause pneumonia, though it ranges from the fatal kind to much milder, so-called “walking” pneumonia. Luckily, our immune systems can detect Legionella and send white blood cells to the infected lung.

Plus, most patients respond well to antibiotics. Legionella doesn’t rely on bugs to spread it, like the first two parasites. Instead, it lives in water: places like stagnant freshwater ponds, plumbing systems, or stale hot tubs.

Legionella has a particularly nasty feature where it’s resistant to chlorine and heat, our usual methods of sterilizing water. It does need a host to reproduce, but that can come in the form of amoebas sharing the water with it. And it spreads when people breathe in tiny water droplets that harbor the bacteria.

That could mean anything from an outdoor mister to an indoor humidifier. The important thing is that you inhale this water, not drink it. Once in your system, Legionella sticks to its host cell and triggers the cell to take it up.

But it’s less picky than our first two examples about what kind of cell it infects. And that means it probably has multiple ways of getting in. Lung cells, in particular, engulf the bacterium in a compartment where it would normally be destroyed.

But somehow, the bacterium keeps the destruction part from ever happening, and lives there quite happily instead. Meaning until we work out better strategies for keeping Legionella out of our water, it’s going to keep finding ways to call our lung cells home.

Number four on our list is one of the least dangerous but most common sexually transmitted infections. Chlamydia trachomatis, or affectionately, just chlamydia. This parasite spreads very efficiently. It’s the most commonly reported bacterial STI in the U.S., though many people who have it don’t show symptoms.

Chlamydia is a bit of an oddball in that it’s a lot less independent than some other parasites. They can’t swim or move on their own, and they lack the ability to produce their own energy, so they depend on their host cell for just about every vital function. So the chlamydia bacterium floats around in an inert form until it attaches to a host cell.

Then the host cell wraps up the invader in a little package called an inclusion, similar to if the cell were trying to destroy it. Cells have these bodies called lysosomes that are full of destructive enzymes. Now, usually, we’d expect the little intracellular inclusion bundle to fuse with a lysosome and let those enzymes disintegrate the invader.

And if the host detects something really bad happening, it can even destroy itself along with the parasite in the process. Chlamydia not only avoids destruction, but manipulates its host cell to hijack its energy sources. They do that by using a variety of proteins to communicate with their host from inside their cosy little inclusions.

After the bacterium establishes itself in the inclusion, it starts replicating and matures, and eventually bursts out from the host cell to start the process over again. From there, chlamydia spreads to new people through a different method, including sexual activity or childbirth. Luckily, chlamydia is pretty easy to cure with a round of antibiotics.

Either way, if you’re sexually active, make sure you’re routinely getting tested.

Next up is microsporidians, an entire phylum of single-celled organisms. And they’re exactly what the name implies — parasites with tiny spores that they use to invade host cells.

But these aren’t bacteria — current research suggests they’re either a type of fungus, or something closely related to fungus, we’re not sure. Now, most humans that contract a microsporidian don’t show any symptoms — though not in all cases.

And they tend to show up more frequently in patients with compromised immune systems, like folks with HIV or those who are undergoing radiation therapy. And because this is a big and diverse group of parasites, an infection can cause different symptoms depending on the type of microsporidian and what body part it infects.

Which includes the gut, lungs, and eyes, to name a few, which is lovely. And they have a really fun way of getting into cells. 
When a spore encounters a favorable environment, it starts to germinate. Then, it grows a tiny harpoon called a polar tubule which it uses to spear the unfortunate host cell. it uses the polar tubule to inject reproductive material, which then goes on to eventually develop into a whole bunch of mature spores, which burst out of the host cell and start the process over again. 
Microsporidians don't limit themselves to humans; they can live in all sorts of cell types and a bunch of different animals, as well as other organisms. 
But, so might person-to-person contact, inhalation, ingestion, and direct contact with your eyes. we actually don't understand how it's transmitted all that well. basically, these things are everywhere. Plus, you might have one right now and not even know it. Cool. Terrifying. Luckily, they aren't that harmful. 
Finally, we just couldn't leave out this last parasite on our list: Polypodium. Unlike the other parasites on our list, this one wants nothing to do with humans. It exclusively affects the egg cells of sturgeons and paddlefish. We saved the best for last, though, because these parasites aren't bacteria or single-celled organisms. Scientists think they're related to jellyfish. Polypodium live in the fish's egg cells for years before bursting out when the fish spawns, which then destroys that egg cell. What a bummer for that fish. They grow all those egg cells only to realize they've been hosting a Polypodium. Scientists have had a hard time classifying polypodium, but they're fairly confident they're Cnidarian, belonging to the same group as jellyfish. They make that call partially because it has those trademark stingers, or Nematocysts. While they can get around in freshwater with no problem, what's peculiar is how these critters grow up, which is also what makes them parasites. The free-living parents appear to deposit the next generation of parasites on the skin of their fishy hosts. But exactly how they get into the eggs of those fish is kind of a mystery. 

When they first show up they look like a tiny cell with two nucli, one bigger than the other.  But then that smaller nucleus surrounds itself with some more cellular material and becomes a sort of cell within a cell.  We're still inside of a fish egg here; meaning we have a cell, within a cell, within a cell. 

The innermost polypodium cell replicates into a more devoloped larva, the outer cell serves to protect it.  At this pont the larva is a long slender organism with a bunch of tenticles tucked inside it's pod.  When it's ready the polypodium turns itself inside out exposing its tenticles.  It bursts out of the host cell and swims out into the open water. 

Best of all that baby polypodium fragments into mutiple free living adults, meaning this thing takes turns reproducing asexually and doing it's best alien impression, in fish eggs.  At this point the adult polypodium lives on it's own and continues to devolop it's sex organs until it's ready to parasitize another fish.  Starting the process all over again.

You might be reasonably horrified of these guys by now, but you're not a sturgeon, so you're fine.  

These six examples are just a few of the multitude of parasites who see the insides of other creatures cells as free real estate.  Some can cause intense human disease, while others only infect specific fish eggs.

Either way, a better understanding of how these parasites live will give us greater insight into how to prevent disease and improve life for everyone.  And while we are talking about tiny things, we want to give a shout out to our new sister channel produced by complexly.  Journey to the microcosmos.  Journey to the microcosmos is all about bringing you closer to the world of very very small things.  All incredible microscopic footage is provided by James Whise, pair that with meditative naration and music by Hank Green and Andrew Whong respcectively and you've got what we like to think is the most relaxing educational channel ever.
Check it out at the link in the discription, and thanks for watching!

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