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 the 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's still 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.

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 this 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 our 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 cells to take it up.

But it's less picky than our first two examples about what kind of cells 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 the host from inside their

cosy little inclusions.

After the bacterium establishes itself in the inclusion,

it starts replicating and matures, eventually bursting out from the host cell

to start the process over again.

From there, chlamydia spreads to new people through a few different methods,

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 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.

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 in a bunch of different kinds of animals,

as well as other organisms.

Which means contact with other animals is one way these things might spread --

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 know it.

Cool.

Terrifying.

Luckily, they usually aren't too harmful.

Finally, we just couldn't leave out the last parasite on our list, Polypodium.

Unlike the other parasites on our list, this one wants nothing to do with humans.

It exclusively infects 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 fishes' 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 Polypodium.

Scientists have had a hard time classifying Polypodium,

but they're fairly confident it's a cnidarian -- belonging to the same group as jellyfish.

They make that call partly 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 the fish is still kind of a mystery.

When they first show up, they look like a tiny cell with two nuclei, 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 developed larva.

The outer cell serves to protect it.

At this point, the larva is a long, slender organism with a bunch of tentacles tucked

inside its body.

When it's ready, the polypodium turns itself inside out,

exposing its tentacles, bursts out of the host cell, and swims out into open water.

Best of all, that baby Polypodium fragments into multiple free-living adults.

Meaning this thing takes turns reproducing asexually,

and doing its best Alien impression in fish eggs.

At this point, the adult Polypodium lives on its own and continues to develop its sex

organs

until it's ready to parasitize another fish —

starting the process all over.

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 some 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're talking about tiny things,

we want to give a shout-out to our new sister channel produced by Complexly: