Chapter 13: Viruses, Viroids, and Prions

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Welcome to the Deep Dive.

Okay, picture this.

You're at the doctor's office.

Maybe your kid's got a fever, runny nose.

You're already thinking, okay, here we go, antibiotics again.

But then the doctor says, nope, antibiotics won't help this time.

Why?

Well, because sometimes the bug isn't a bacterium, it's something way, way smaller,

a virus.

And that's really our jumping off point today.

For ages, these microscopic things were just baffling, couldn't see them, but they caused massive problems.

So today we're digging into some really tricky subjects in microbiology, viruses, viroids and prions.

Right.

And our mission here for this Deep Dive is to pull out the key ideas, the really surprising stuff and the critical points about these tiny entities.

We're leaning hard on a great chapter from microbiology and introduction, the 13th edition.

Yeah, it's a solid source.

We want to give you that shortcut.

I know, so you really get how these things work, how scientists figure them out and the huge impact they have on us, on the whole planet, really.

It's quite a story.

It goes way back, late 1800s.

Scientists like Mayer Iwanowski, they found these filterable agents, something causing tobacco mosaic disease passed right through bacteria filters.

So smaller than bacteria, much smaller.

Exactly, but they couldn't see them.

Not until the electron microscope came along in the 1930s, then finally we got to look at their actual form.

And ever since then, right up to things like HIV, SARS -Co, these tiny structures just keep messing with our definition of life itself.

Which brings us to the big question, are viruses actually alive?

That is surprisingly tricky, honestly.

Outside a host cell, a virus is just inert chemicals, basically, not alive.

Just sitting there.

Right, but the second it gets inside a living cell, its genes switch on, it starts multiplying, it acts alive.

And, you know, clinically speaking, they cause disease.

So yeah, they're treated as alive in that context.

It's a bit philosophical, but with real practical consequences.

Okay, so it's this weird in -between state.

What really makes a virus fundamentally different from, say, a bacterium, then?

What are the defining features?

The core things, the real takeaways, there are four.

First, they have only one type of nucleic acid, it's either DNA or RNA, never both, unlike cells.

Okay, DNA or RNA.

Got it.

Second, they have a protein coat, the capsid, maybe an outer envelope too.

Third, and this is absolutely key, they are obligatory intracellular parasites.

They have to get inside a living cell to multiply.

They use the host's own equipment.

Total dependence.

Total dependence.

And fourth, they make specialized structures that help get their nucleic acid into the next host cell.

So that absolute reliance on the host cell, that's the big difference.

Bacteria can often do their own thing, make proteins, generate energy.

Exactly.

Viruses, nope.

They lack the enzymes for making proteins for generating ATP, completely reliant.

And that, right there, is why making antiviral drugs is so incredibly hard.

Because anything that stops the virus might also mess with our own cells.

Precisely.

Hit the virus, you often hit the host.

It's a really fine line to walk, finding drugs that are effective, but not too toxic.

And they're picky eaters too, right?

They don't just infect anything, this host range idea.

That's right.

Most viruses are pretty specific.

They target certain cell types in just one kind of host species, so you've got viruses for insects, for plants, for fungi, for us, and even for bacteria.

Those are the bacteriophages, right?

Phages, yep, bacteriophages, or phages for short.

And while many are super specific, some do manage to jump species barriers.

Influence is a classic example, we'll get into that.

But that specificity, it's actually fueling new interest in phage therapy.

Using viruses to fight bacteria.

Kind of ironic, isn't it?

But yeah, using these highly specific phages to kill off bacterial infections without harming our own cells, it's an old idea getting a fresh look.

That is interesting.

Now, when you say tiny, how tiny are we talking?

Compared to a bacterium, for instance?

Much smaller, generally.

Most are between 20 ,000 nanometers.

You absolutely need an electron microscope to see the structure of most of them.

Bacteria are giants by comparison.

Okay, here's something that sounds kind of wild.

The human virome, what is that?

Should we be worried?

Well, it sounds alarming, but it's the fact that our bodies are literally teeming with viruses.

We're talking trillions of permanent infectious viruses living in and on us.

Trillions, seriously?

Trillions.

But hold on, the vast majority are those bacteriose ages,

the ones that infect bacteria.

They're a huge part of our microbiome, especially in places like our mouth, our gut mucus.

Okay, so they're mostly targeting the bacteria in us, not us directly.

Largely, yes.

Yeah.

And it's really complex and often beneficial.

There's this idea called kill the winter phages, can prevent potentially harmful bacteria from taking over, like tiny bodyguards.

To get a balance.

Exactly.

And then there's kill the competition.

Some friendly gut bacteria, like certain innerococcus, actually release phages to specifically kill off rival bacteria.

So they're using viruses to protect their own turf within our microbiome.

It's this intricate ecosystem.

Wow.

Okay, let's zoom in on the virus particle itself.

Outside the cell, you called it a virium.

What's the basic structure?

Right.

The virium is the complete infectious particle.

When we classify viruses, it's mainly based on two things.

What kind of nucleic acid they have, gene A or RNA, remember never both, and the structure of their protein code.

And that nucleic acid,

it's not always simple.

Not at all.

It can be single stranded DNA, double stranded DNA, single stranded RNA, double stranded RNA.

It can be a linear molecule or circular or even broken up into segments like the flu virus has.

Lots of variation.

And this genetic core is protective.

Yes.

By the capsid.

That's the protein shell.

It's made of smaller protein subunits called capsimers.

The arrangement of these capsimers gives the virus its shape, and it's actually determined by the viral nucleic acid itself.

And some have an extra layer, an envelope.

Some do, yes.

The envelope is typically a layer of lipids, proteins, and carbohydrates surrounding the capsid.

Interestingly, they often steal this layer from the host cell's own membrane as they're exiting the cell.

Sneaky.

And those little things sticking out.

The spikes.

Ah, yes.

The spikes.

Those are usually glycoprotein structures, protein plus carbohydrate.

They're absolutely critical for the virus.

They're what attaches to the receptor sites on the host cell.

Think of them like keys fitting into specific locks on the cell surface.

So that's how they target specific cells.

Exactly.

And they're useful for us scientists, too, for identification.

For instance, the influenza virus uses its spikes to clump red blood cells together.

That's called hemagglutination.

We can actually use that reaction in lab tests to identify flu strains.

And spikes are related to why we can get the flu over and over.

They are.

The genes coding for these spikes, particularly in influenza, mutate quite often.

Small changes mean the antibodies we developed from a previous infection or vaccine might not recognize the new version effectively.

So the virus changes its key and our immune system's lock doesn't fit anymore.

That's a good analogy.

This gradual change is called antigenic drift.

It's why the flu vaccine needs updating frequently.

And of course, some viruses don't have an envelope at all.

They're just the nucleic acid protected by the capsid.

We call those non -enveloped viruses.

OK, so we have the components.

Nucleic acid, capsid, maybe an envelope and spikes.

What about the overall shapes?

Are they all just little balls?

Oh, no.

There's quite a bit of variety in their morphology.

You have helical viruses, which look like long rods.

They can be rigid or flexible.

The capsid is kind of like a hollow tube with the nucleic acid coiled inside.

Rabies and Ebola viruses are examples.

Ebola has that really distinct long filamentous shape.

Right.

I've seen pictures of that.

But you have polyhedral viruses.

Polyhedral just means many -sided.

Most of these are icosahedral.

That's a shape with 20 triangular faces and 12 corners.

Think of a geodetic dome almost.

Adenoviruses, which cause some colds.

And polioviruses look like this.

Very geometric.

Geometric invaders.

What about enveloped ones?

Enveloped viruses often end up looking roughly spherical, just because the envelope kind of bags everything up, whether the capsid inside is helical or polyhedral.

Influenza is enveloped helical.

Herpes viruses are enveloped polyhedral.

And then there are the really weird ones.

Yeah, the complex viruses.

They don't neatly fit the helical or polyhedral categories.

Bacteriophages are the classic example.

They often look like little, leaner landers, right?

A polyhedral head containing the DNA attached to a helical tail sheath with tail fibers for landing on the bacterium.

Like tiny spacecraft.

Pretty much.

And pox viruses are also complex.

They're kind of brick -shaped with multiple coats and no clear capsid.

So with all this diversity, how do scientists actually classify them, put them into families?

It can't just be based on the disease they cause, right?

No, that used to be the way.

But it's too misleading because different viruses can cause similar symptoms.

Now classification relies heavily on, well, their fundamental properties,

the type of nucleic acid, DNA or RNA, single or double -stranded, their strategy for replication, their genetic makeup, genomics, and their structure or morphology.

More reliable characteristics.

Much more.

And there's a naming system.

Genus names end in virus, family names in viridae, and order names in ales.

Let's connect this to something really critical like flu pandemics.

You mentioned influenza earlier.

Right.

Influenza A.

It's a prime example of why understanding viral families and characteristics is crucial.

These viruses infect lots of animals.

Birds, pigs, humans,

and sometimes they jump species.

How does that happen?

Well, pigs are interesting because their cells have receptors for both bird flu viruses and human flu viruses.

They can act as mixing vessels.

Uh -oh.

Yeah.

The flu virus genome isn't one long strand.

It's in eight separate RNA segments.

If a pig gets infected with a bird flu and a human flu at the same time inside a single cell, those segments can get shuffled around, reassorted when new viruses are assembled.

So you can get a brand new virus with a mix of bird and human flu genes.

Exactly.

This is called antigenic shift.

It's major sudden change, unlike the gradual drift we talked about with spikes.

This reassortment can create a novel virus strain that human immune systems have never seen before.

Potentially leading to a pandemic.

The 2009 H1N1 swine flu was one of these reassortant viruses, a mix of genes from pig, bird, and human flu strains.

That's, yeah, that's a huge deal for public health, understanding that mixing potential.

And looking across the viral families, you see the range, coronavirus with SARS and MERS, retroviridae with HIV, all classified by these core properties.

Right.

The classification reflects fundamental biology, which helps predict behavior.

Okay.

So let's get into the action.

How do these viruses actually multiply?

That's the core of their existence, right?

Hijacking the host cell.

That's their whole game plan.

Obligate intracellular parasites.

They must get inside to make copies.

And there's this interesting phase called the eclipse period right after infection.

The eclipse period?

Yeah.

During this time, if you were to break open the infected cells, you wouldn't find any complete infectious virions.

Just parts.

The virus has basically disassembled itself to start the replication process.

The complete virus temporarily disappears.

Taking itself apart to rebuild.

Let's start with those bacteriophages again.

You mentioned two cycles, the little lytic and lysogenic.

What's the little lytic one about?

The lytic cycle is the quick and destructive path.

It ends with the host cell bursting lysis.

Think of it in five stages, using a T -even phage infecting E.

coli as an example.

First, attachment.

The phage uses its tail fibers to grab on to specific receptors on the bacterial cell wall.

Like docking.

Exactly.

Second, penetration.

The phage releases an enzyme, phage lysozyme, that chews a little hole in the cell wall.

Then the tail sheath contracts and it injects its DNA directly into the bacterium like a syringe.

The protein coat, the capsid, stays outside.

Just the DNA gets in.

Okay, then what?

Third is biosynthesis.

This is where the takeover happens.

The phage DNA directs the host cell to start making viral components, more phage DNA, capsid proteins, enzymes.

The host's own DNA often gets chopped up.

This is the main part of that eclipse period.

The cell becomes a virus factory.

Pretty much.

Fourth stage is maturation.

The newly made phage DNA and capsids spontaneously assemble into complete new virions.

Hundreds of them, usually.

Self -assembly.

Cool.

And finally, fifth stage,

release.

The phage directs the production of more lysozyme, which breaks down the bacterial cell wall from the inside out.

The cell ruptures, releasing all those new phages to go infect neighboring cells.

End of story for that bacterium.

Brutal.

Okay, so what about the lysogenic cycle?

You said it was sneakier.

Right.

The lysogenic cycle is different.

Here, the phage DNA doesn't immediately take over.

Instead, it integrates itself into the host bacterium's own chromosome.

It hides inside the host's DNA.

Exactly.

This integrated phage DNA is called a profish.

And it just sits there, passively replicating along with the bacterial DNA every time the bacterium divides.

The host cell isn't harmed, initially.

So it's dormant, latent.

Does it ever wake up?

It can, yes.

Certain environmental triggers UV light.

Some chemicals can induce the profish to pop back out of the bacterial chromosome.

And once it's excised, it usually enters the neolithic cycle, leading to destruction and release of new phages.

Ah, so it can switch strategies.

What are the consequences of this lysogeny, this hiding strategy?

There are three really important outcomes.

One, the lysogenic bacterium becomes immune to being reinfected by the same type of phage.

The profish kind of blocks the door.

Okay, self -protection.

Two, and this is fascinating and clinically very relevant,

phage conversion.

The host bacterium might actually exhibit new properties because of the profish genes it's carrying.

Like what kind of properties?

Like the ability to produce toxins.

The bacteria that cause diphtheria, botulism, and even cholera.

They only produce their dangerous toxins if they are infected with a specific lysogenic phage carrying the toxin gene.

Wait, so the virus gives the bacteria its weapon?

In these cases, yes.

A harmless bacterium can be turned into a pathogen by its resident profish.

It's a huge factor in bacterial virulence.

That is genuinely mind -blowing.

Okay, what's the third outcome?

The third is specialized transduction.

When the profish exits the bacterial chromosome, sometimes it accidentally takes a little piece of the adjacent bacterial DNA with it.

When this phage then infects a new bacterium, it carries that bacterial gene along, potentially giving the new host a new trait, like the ability to metabolize a certain sugar.

It's a way bacteria can swap genes mediated by viruses.

Okay, that covers the bacteria viruses.

What about animal viruses?

How do they multiply in our cells?

Siminal process.

The basic steps are there.

Attachment, entry, encoding, biosynthesis, maturation, release.

But the details, especially entry and biosynthesis, can be quite different.

Let's start with attachment.

Is it still specific?

Very specific.

Animal viruses have attachment sites, often those spikes we talked about, or specific capsid proteins that bind to complementary receptor sites on the host cell's plasma membrane.

These receptors are usually proteins, or glycoproteins, the cell needs for its normal functions.

So the virus exploits the cell's own doorway proteins.

Exactly.

And differences in these receptors explain why some people might be naturally resistant to certain viruses.

If you lack the right receptor, the virus can't get in.

Okay, they've attached.

How do they get inside?

Not usually injection, like phages, right?

Usually not.

One common way is receptor -mediated endocytosis.

The host cell membrane basically folds inward and engulfs the virus particle, bringing it inside within a vesicle.

The cell actively pulls it in.

In a way, yes, tricked by the virus binding to the receptor.

The other main method, particularly for enveloped viruses, is fusion.

The viral envelope literally fuses with the host cell's plasma membrane, sort of like two soap bubbles merging and dumps the capsid directly into the cytoplasm.

Okay, inside now.

Then it has to get its genetic material out of the capsid.

Yes, that's uncoating.

The separation of the viral nucleic acid from its protein code.

This can happen in different ways, sometimes using enzymes from the host cell's own lysosomes, sometimes triggered by acidity in vesicles, or maybe using viral enzymes.

This is when the virus disappears during that eclipse period we mentioned.

Okay, nucleic acid is free.

Well, biosynthesis.

Yeah.

This is where it gets complicated, depending on DNA versus RNA, right?

Absolutely.

This is the most diverse part of the cycle.

Let's take DNA viruses first.

Most of them replicate their DNA in the host cell's nucleus using viral enzymes.

They synthesize their capsid proteins out in the cytoplasm using host ribosomes.

Then those proteins travel back into the nucleus for assembly.

Why the nucleus?

Because that's where the host cell's DNA replication machinery and enzymes are.

Viruses like adenoviruses, herpesviruses, HPV.

They follow this pattern.

Poxviruses are a weird exception.

They do everything on the cytoplasm.

And hepatitis B is super weird.

A DNA virus that uses reverse transcriptase, an enzyme usually associated with RNA viruses.

Okay, so DNA viruses mostly use the nucleus.

What about RNA viruses?

Most RNA viruses do their whole replication cycle out in the cytoplasm.

The key thing here is an enzyme called RNA -dependent RNA polymerase.

Host cells don't normally have this enzyme.

They don't make RNA from an RNA template.

So the RNA viruses either have to carry the enzyme with them into the cell or synthesize it early on.

And how that enzyme works depends on the type of RNA.

Exactly.

For viruses with what we call a plus strand or sense strand, RNA genome like poliovirus.

Hepatitis A, their RNA can act directly as mRNA.

It gets immediately translated by host ribosomes to make viral proteins, including that RNA polymerase needed to copy the genome.

So the genome is the message.

Right.

But for anti -strand or anti -sense strand, RNA viruses like rabies, influenza, their genome can't be translated directly.

They first have to use their RNA polymerase to transcribe their strand RNA into a complementary plus strand.

And that plus strand then serves as mRNA.

A bit more complicated.

And double -stranded RNA.

Like reoviruses, which cause rotovirus.

Yeah, they have double -stranded RNA.

They replicate in a way where mRNA is produced from the RNA genome inside the capsid.

Yep.

And then released into the cytoplasm.

Then there are the really unique ones, retroviruses.

Like HIV, RNA genome, but involves DNA.

How does that work?

It's a fascinating process.

Retroviruses, like HIV, carry a special enzyme called reverse transcriptase.

When the virus enters the cell and uncoats, this enzyme uses the viral RNA genome as a template to synthesize a complementary strand of DNA.

RNA back to DNA.

That's backwards.

Exactly why it's called reverse transcriptase.

It then makes a complementary DNA strand, forming double -stranded DNA.

This viral DNA then travels to the nucleus and gets integrated into the host cell's own chromosome using another viral enzyme called integrase.

So it becomes part of our DNA.

Yes.

This integrated viral DNA is called a provirus.

And here's a crucial difference for the bacterial provache.

The provirus never comes back out.

It becomes a permanent part of the host cell's genetic material.

Wow.

So it can just hide there indefinitely.

It can remain latent, just being copied along with the host DNA every time the cell divides.

Or it can be transcribed back into viral RNA, some of which acts as mRNA to make viral proteins, and some becomes the genome for new viruses.

Because it's integrated, it's protected from the immune system and many antiviral drugs.

It makes infections like HIV lifelong.

That explains a lot.

Okay.

After all this biosynthesis, DNA, or RNA,

the parts are made.

How do they assemble and get out?

Maturation and release.

Right.

Maturation is the assembly of the capsid proteins and nucleic acid into complete virions.

Often, this happens spontaneously.

Then release.

For non -developed viruses, it's usually pretty destructive.

They build up inside the cell until it bursts, releasing them, but killing the host cell.

Lysis again?

Yep.

But enveloped viruses typically get out by budding.

The assembled capsid pushes through one of the host cell's membranes.

It could be the plasma membrane, nuclear membrane, ER membrane, and takes a patch of that membrane with it, forming the envelope.

So it cloaks itself in the host's membrane.

Does that kill the cell?

Not necessarily, or at least not immediately.

Budding can be a slow, continuous process, allowing the host cell to survive for a while, shedding viruses.

This contributes to persistent or chronic infections.

Okay, this multiplication explains how viruses cause acute illness, but you mentioned earlier a link to cancer.

That seems like a totally different kind of problem.

It does, but the connection is very real.

It was actually first noticed over a century ago in chickens with leukemia and sarcoma caused by viruses.

But it took a long time to be recognized in humans.

Why the delay?

Several reasons.

Cancer often develops long after the initial viral infection.

Not everyone infected with a cancer -causing virus actually gets cancer.

And cancer isn't typically contagious like, say, the flu, so it didn't fit the standard infectious disease model.

So how do viruses cause cancer?

What's the mechanism?

Does the virus carry a cancer gene?

Sometimes, but more often it's about activating genes already present in our own cells.

We all have genes called proto -oncogenes that regulate normal cell growth and division.

If these genes get mutated or activated inappropriately by radiation, chemicals, or by certain viruses, they can become oncogenes, driving uncontrolled cell growth, which is cancer.

So the virus flips a switch that's already there.

Often, yes.

This was a Nobel Prize -winning discovery, realizing that oncogenes were often just altered versions of our normal genes.

Oncogenic viruses, or cancer -causing viruses, often work by integrating their genetic material into the host cell's DNA.

This integration can disrupt normal gene regulation or directly activate a proto -oncogene.

And this leads to changes in the cell?

Yes.

The cell becomes transformed.

It loses normal growth control, changes shape, and may display unique markers on its surface called tumor -specific antigens.

What are some major examples of viruses linked to human cancers?

There are several significant ones.

Human papillomavirus, HPV, is probably the most well -known now.

Certain types are responsible for almost all cervical cancers, plus many anal, throat, and other cancers.

That's why the HPV vaccine is such a crucial public health tool.

Definitely others.

Extinbavirus, EBV, the virus that causes mono, is linked to Birkitt's lymphoma, especially in certain parts of Africa, and also nasopharyngeal cancer.

Hepatitis B virus, HBV, and hepatitis C virus, HCV, are strongly linked to liver cancer.

These are DNA viruses, HPV, and RNA viruses, HCV, that cause chronic liver inflammation, increasing cancer risk over time.

And any RNA viruses that directly cause cancer, besides hep C promoting it?

Yes, the main examples are retroviruses, specifically human T cell leukemia viruses, HTLV1 and HTLV2.

Their ability to integrate that provirus into the host genome is key to how they disrupt cell regulation and cause certain types of leukemia and lymphoma.

It's sobering how involved viruses are, but then there's the flip side using viruses against cancer, oncolytic viruses.

Yes, that's a really exciting area of research.

Oncolytic viruses are viruses that are either naturally inclined to infect and kill cancer cells more readily than normal cells, or they've been genetically engineered to do so.

How do they work?

They can directly kill the tumor cells by replicating inside them and causing lysis.

Or their presence can trigger a strong immune response from the host, which then attacks the cancer.

The FDA has approved one called Imlegic, a modified herpes virus, for treating melanoma.

It's still early days, but very promising.

Okay, shifting gears slightly.

We often hear about latent infections, like cold sores.

How does that differ from a persistent or chronic infection, like HIV?

Good distinction to make.

In a latent infection, the virus essentially goes dormant inside host cells for potentially long periods.

There's no active disease, no virus production.

But then, triggers like stress, sunlight, other illnesses can reactivate the virus, causing symptoms to suddenly appear.

Like cold sores from herpes simplex virus, HHV1 or 2, or shingles from the chicken pox virus, varicella virus or HHV3, reactivating years later.

Exactly.

The virus hides out, often in nerve cells, and then reactivates.

In persistent infections, though, the virus isn't truly dormant.

It continues to replicate, usually at a low level, over a long period, often months or years.

Detectable virus gradually accumulates, and disease symptoms develop slowly.

So,

HIV leading to AIDS, or chronic hepatitis B, or C potentially leading to liver cancer, those are persistent.

Correct.

Measles virus causing a rare but fatal brain disease, called SSTE, years after the initial infection, is another example of a persistent infection.

The key difference is that continuous presence and accumulation of virus in persistent infections, versus the apparent absence, then sudden reappearance in latent ones.

Let's circle back to that hepatitis outbreak example, Tina's case.

Identifying the specific virus was critical.

Absolutely critical.

Hepatitis A, B, and C are all inflammation of the liver, but caused by very different viruses with different structures, genomes, and transmission routes.

Hepatitis A is caused by the picorna virus, non -enveloped, plus strand RNA usually spread fecal orally, like through contaminated food or water, common in outbreaks.

Like Tina's town experienced.

Exactly.

Hepatitis B is a hepatinivirus -enveloped, DNA virus using reverse transcriptase spread through blood, sexual contact, hepatitis C is a phleifovirus -enveloped, plus strand RNA mainly spread through blood.

Knowing the outbreak involved dozens of people pointed away from B and C, and strongly towards A.

And knowing it was hep A allowed for specific actions.

Yes.

Public health officials could focus on contaminated food sources, advise on hygiene, and crucially recommend the hepatitis A vaccine, or immunoglobulin injections for people potentially exposed, helping to stop the outbreak.

You need to know your enemy.

Absolutely.

Now we focused a lot on animal viruses.

What about plants?

Do they get viruses too?

They certainly do.

Plant viruses cause huge economic losses in agriculture worldwide.

Structurally, they're similar to animal viruses, helical polyhedral with RNA or DNA genomes.

How do they infect plants?

Plants have tough cell walls.

That's the key difference.

They can't just detach, infuse, or get endocytosed like animal viruses often do.

They need a way through that wall.

So they usually enter through wounds, like from physical damage or pruning, or they hitch a ride with parasites that feed on plants, like insects, nematodes, or fungi.

The parasite essentially injects the virus past the cell wall.

Okay, makes sense.

Now let's go even simpler, structure -wise.

Viroids.

You said there's just naked RNA.

That's right.

No protein coat at all.

Just short, circular pieces of single -stranded RNA, only about 300 -400 nucleotides long.

Kind of amazing they survive.

How do they survive without a protective coat?

The RNA molecule folds back on itself extensively, creating lots of base pairing.

This forms a very compact, stable, kind of rod -like structure that gives it some protection from cellular enzymes.

And how do they cause disease if they don't even code for proteins?

That's the really puzzling part.

The leading theory is that they interfere with the host cell's own gene regulation, possibly through a process called RNA silencing.

They might mimic or disrupt the small RNAs the plant uses to control its own genes, leading to developmental problems.

Like the potato spindle tubervuroid, causing stunted growth.

Exactly.

That was the first one discovered.

Causes major crop losses.

And there's a related entity, virusoids, which are viroid -like RNAs that actually do get packaged inside the protein coat of a different helper virus.

They need that helper virus to replicate and spread.

Hepatitis D in humans might be something like a virusoid.

It requires co -infection with hepatitis B.

Intriguing.

Any idea where these weird little RNAs came from?

One hypothesis points to introns, those non -coding sequences within eukaryotic genes that get spliced out.

Viroids share some structural similarities with introns, leading to speculation that maybe they're escaped introns.

It also makes you wonder if there might be undiscovered viroids causing diseases in animals.

Okay, from naked RNA to just protein.

Prions.

This one still feels like science fiction.

Infectious protein.

It really turned biology on its head when Stanley Prusiner proposed it in the 1980s and won a Nobel Prize for it.

Prions are infectious agents composed solely of protein.

No DNA, no RNA.

And they cause these horrible brain diseases.

Spongiform encephalopathies.

Yes.

Transmissible Spongiform Encephalopathies.

TSEs.

They cause progressive neurodegeneration.

The brain develops characteristic holes, making it look spongy under a microscope.

Like Screpey in sheep, mad cow disease, BSE in cattle.

Exactly.

And in humans, there's Krulzfeldt -Jakob disease, CJD, Kuru, linked to cannibalistic rituals in New Guinea, Gerstmann -Streusler -Schranker syndrome, and Fatal Familial Insomnia.

What's bizarre is how they spread.

Some seem inherited, others infectious.

That's the paradox.

TSEs can arise from a genetic mutation in the gene coding for the prion protein, inherited CJD, Fatal Familial Insomnia.

They can occur sporadically, seemingly out of nowhere, most CJD cases.

And they can be transmitted through contaminated food, like BSE jumping to humans as variant CJD, contaminated surgical instruments, corneal transplants, growth hormone injections.

So how does a protein with no genes replicate itself and cause disease?

It's a process of conversion or corruption.

We all have a normal cellular protein called PRPC, found mostly on the surface of nerve cells.

The infectious form, called PRPSC,

has the same amino acid sequence but is folded into a different abnormal shape.

Misfolded.

Exactly.

And the crucial, terrifying part is that when this misfolded PRPSC encounters a normal PRPC molecule, it somehow induces the normal protein to refold into the abnormal PRPSC shape.

Like a template for misfolding?

Pretty much.

So one abnormal prion protein triggers a chain reaction, converting more and more normal protein into the misfolded infectious form.

These PRPSC molecules are very stable, they resist being broken down by the cell, and they aggregate together, forming clumps or plaques.

And that accumulation kills the nerve cells.

That's the result.

The accumulation of these aggregates disrupts cell function and eventually leads to cell death, creating those vacuoles or holes in the brain tissue and causing the devastating neurological symptoms.

Diagnosis is often confirmed post -mortem by finding these plaques.

It's a truly unique mechanism of disease.

Wow.

We have definitely covered a lot of ground today.

From the basic structure of viruses, these tiny hijackers, through their diverse ways of multiplying, their surprising links to cancer, and then onto these even stranger entities, viroids and prions.

It's a whole hidden world, isn't it?

These entities that blur the lines of life, completely dependent on host cells, yet capable of causing so much disruption, disease.

But also, as we saw with the virome, potentially playing complex roles in our own biology.

You really do get a much clearer picture now.

So next time you hear about the flu or a new vaccine, or even one of these rare neurological diseases, you'll have a better sense of the incredible intricate and often invisible biological battles being waged.

Yeah.

Understanding the how behind it all makes a big difference.

And here's something to chew on.

If viruses are masters of hijacking cellular machinery, and prions show that just a misfolded protein can be infectious,

what other fundamental biological processes could be vulnerable?

Are there maybe other kinds of microscopic manipulators, other forms of infection or pseudo life out there we haven't even conceived of yet?

It's a fascinating question.

The microbial world is constantly surprising us.

There's always more to learn, more weirdness to uncover.

That's what makes it so compelling.

Absolutely.

Well, thank you for joining us on this Deep Dive.

We really hope exploring viruses, virides and prions has given you some valuable insights, maybe sparked even more curiosity.

Stay curious, everyone, and we'll see you next time for another Deep Dive.