Chapter 38: Viral & Prion Diseases in Humans

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Welcome to the Deep Dive.

You know, when you look back at global health history, it's hard to overstate the impact of HIVs.

Absolutely, especially before 1996.

It was devastating.

Right.

But then those combination antiretroviral drugs came along and groups like UNAIDS started pushing this idea of treatment as a human right.

And that really changed everything, not just for HIV, but it reshaped global health priorities.

Yeah.

And it massively boosted our understanding of the immune system itself.

One virus did all that.

It's true.

HIV really forced us to grapple with how these global crises, you know, emerge and how we respond.

And that's kind of the thread we're pulling today.

OK, so let's unpack this.

This Deep Dive is all about the major human viral and prion diseases.

We're looking at sources that categorize them by how they spread.

Right.

So airborne, arthropod born, direct contact, food and water, zoonotic.

It's about the transmission route.

And our mission here is really to get into the core microbial mechanisms like the how.

How do these pathogens actually work?

How do they get in, survive, cause disease?

We're focusing on those key structure function relationships.

You know, what makes each one tick, basically.

And we're starting with the ones we probably encounter most often.

The airborne ones.

Yeah.

Makes sense.

Yeah.

Through the air.

It's just so efficient.

Your air is just the vehicle carrying these tiny particles from a cough or sneeze.

Exactly.

And our first example here is, well, it's a master class in survival, varicella zoster virus, VZV.

Ah, chicken pox.

Chicken pox, yes.

But that's just the start.

It's an enveloped, double -stranded DNA herpes virus.

Its real trick is latency.

Latency.

So you get chicken pox, you recover, you're immune, but the virus isn't actually gone.

Not at all.

It's hiding.

The viral DNA persists as a circular molecule.

It's called an episome.

An episome.

Okay.

And it just sits there quietly inside the nucleus of specific nerve cells, cranial nerves, dorsal root ganglion neurons.

So it's like this silent tenant in your nervous system.

Pretty much.

It stays dormant until, well, until the host's immune system weakens.

Maybe age, stress, another illness, then it reactivates, travels down those sensory nerves and causes shingles.

That really painful rash, usually in a specific band.

That dermatomal distribution.

Right.

Exactly.

Following the nerve path.

That whole strategy, acute infection, then hiding out for life seems common for herpes viruses.

It is.

But let's switch gears to something known for speed and constant change.

Influenza.

Flu.

Okay.

This is a segmented negative strand RNA virus and its whole game relies on two key proteins on its surface.

These envelope spikes, hemagglutinin or HA and neuraminidase, NA.

HA is for getting in, right?

Sticking to our respiratory cell.

Correct.

Adherence and entry.

But NA, NA is crucial for the flow.

How so?

Well, first it helps the virus get to the cells by breaking down the protective mucus in our airways.

And second, maybe even more importantly, it's the release mechanism.

Release?

Yeah.

Once new viruses are made inside the cell, NA actually cleaves them off the cell surface so they can escape and infect more cells.

Oh, okay.

And this is where the constant change comes in.

The whole drift versus shift thing.

Exactly.

We really need to understand the difference.

Antigenic drift is the slow, steady change.

Okay.

All RNA viruses use an enzyme called RNA -dependent RNA polymerase, RDRP.

And the key thing is this enzyme makes mistakes.

It doesn't proofread well.

So typos in the genetic code.

Sort of, yes.

Little point mutations accumulate over time, changing the HA and NA proteins just enough that our immune system might not recognize them perfectly.

That's why we get seasonal flu outbreaks maybe every few years.

But then there's antigenic shift.

That's the big one, right?

The pandemic trigger.

That's the major upheaval because the influenza genome is in segments, like different chapters in a book.

If two different flu strains, say one from a bird and one from a human, infect the same cell, maybe in a pig.

A mixing vessel, essentially.

Precisely.

Inside that cell, the segments from the bird virus and the human virus can get shuffled and repackaged together.

Segment reassortment.

And out comes a completely new virus.

A hybrid virus with a novel combination of HA and NA that nobody's immune system has seen before.

That's what leads to pandemics, like the H1N1 swine flu in 2009.

And the sources mention rural China as a place where this mixing is more likely.

Yeah.

Often cited because you have pigs, birds, and humans living in really close proximity.

Ideal conditions for that kind of cross -species jump and reassortment.

Before we move off airborne, we should quickly mention measles.

Right.

More bilva virus.

Super contagious.

Causes that characteristic rash.

Copicic spots.

Thankfully vaccine preventable.

And smallpox.

Smallpox is huge.

Variola virus.

The only human disease we've ever eradicated through vaccination.

And why was that possible?

What made smallpox eradicable?

A few key things.

Humans were the only reservoir.

No animals hiding it.

The disease itself was obvious you knew who had it, and people weren't infectious for that long.

Made it possible to track and contain.

No hidden carriers, basically.

Exactly.

Which leads us nicely into the next transmission route, where the environment, or specifically a vector, is crucial.

Arthropod -borne viruses.

Arboviruses.

Transmitted by insects like mosquitoes and ticks.

Yep.

Bloodsuckers.

And what's interesting is the virus actually multiplies inside the insect vector, but usually it doesn't make the insects sick.

Makes them perfect carriers.

But in humans, they can cause a range of things.

Yeah.

Anything from fevers and rashes to really severe encephalitis or hemorrhagic fevers.

And this is where we encounter that counterintuitive mechanism.

Dengue virus.

And antibody -dependent enhancement.

ADE.

Yes, ADE.

It's fascinating and dangerous.

So Dengue virus, BENV, has four main types, or serotypes.

If you get infected with, say, serotype 1, you get sick, you recover, and you develop antibodies.

You're immune to serotype 1 for life.

Makes sense.

But here's the twist.

If you later get infected with a different serotype, like serotype 2, those antibodies you made against serotype 1, they don't effectively neutralize serotype 2.

So they don't stop it.

But why is that bad?

Because they still bind to the serotype 2 virus.

They essentially tag it.

A process called opsonization.

Okay.

Tagging it for destruction, normally.

That's what should happen.

But instead, this tag allows the virus to be taken up more easily by certain immune cells, macrophages,

monocytes.

Wait, the cells that are supposed to clear infections.

Exactly.

Those cells become for the virus.

The virus hijacks the immune response.

This leads to much, much higher levels of virus in the body.

Which causes the really severe form.

Directly leads to Dengue hemorrhagic fever, DHF, or Dengue shock syndrome.

Your own immune memory makes the second infection far worse.

It's a biological betrayal, almost.

Wow.

Okay.

Related to Dengue, another flavivirus, Vika, Z -I -K -V.

That exploded onto the scene around

Right.

Also spread by 80s mosquitoes, like Dengue.

But Zika also gained notoriety for being transmissible through body fluids, particularly semen.

And the major concern there was the congenital risk.

Absolutely devastating.

Congenital Zika syndrome in newborns whose mothers were infected during pregnancy.

Severe microcephaly.

Brain damage.

Just tragic.

How did that outbreak eventually subside?

Well, part of it was that, in populations that hadn't seen it before, immunity built up relatively quickly.

Hurt immunity eventually helped break the transmission cycle.

Okay.

And one more arbovirus example.

West Nile virus,

WNV.

Introduced into the US in 99.

This one helps explain the concept of a dead end host.

Yes.

WNV is carried by Culex mosquitoes, and birds are the main reservoir.

Birds get high levels of the virus, mosquitoes bite them, get infected, bite other birds.

But when a mosquito bites a human.

We get infected, yes.

We can get sick.

But the level of virus in our blood, the viremia, is generally too low for us to pass the virus back to another mosquito that bites us.

So the infection cycle stops with us.

We're a dead end.

The virus hits a wall, transmission -wise.

Before we jumped to direct contact, we skipped over food and water transmission briefly.

Ah, good point.

Important category.

Think hepatitis A and E, HAV and HEV.

Unlike hep B and C, these are typically fecal -oral route.

Contaminated food or water.

Exactly.

And these are usually non -enveloped RNA viruses, which makes them tough.

Like norovirus, the winter vomiting bug.

Perfect example.

Being non -enveloped means they're much more stable in the environment,

resistant to drying, detergents, harder to kill.

So a tiny amount can cause an outbreak.

Very low infectious dose.

Which is why sanitation and clean water are just paramount for controlling these.

You can't rely on the virus just breaking down easily.

Right.

Okay, now let's pivot to direct contact transmission.

And this brings us back to the big one.

HIV AIDS.

We need the molecular details of how it gets in.

Okay.

HIV is a retrovirus.

Its genome is positive strand RNA.

To infect its primary target, the CD4 plus T helper cell, its surface protein, GP120, has to bind to the cell's CD4 receptor.

But that's not enough, right?

There's a second lock.

Correct.

It also needs to bind to a co -receptor.

Usually CCR5 early in the infection, or sometimes CXCR4 later on.

It needs both keys to unlock the door.

Dual key entry.

And once it's bound?

Once it fuses with the cell membrane and the viral core enters the cytoplasm, the defining retroviral step happens.

The reverse transcription.

Yes.

The viral enzyme, reverse transcriptase, uses the viral RNA as a template to synthesize a double -stranded DNA copy.

Making DNA from RNA.

Goes against the central dogma, hence retro.

Exactly.

This DNA copy is called the pro -virus.

And then another viral enzyme, integrase, takes that pro -virus and literally pastes it into the host cell's own chromosomal DNA.

It becomes part of our genome.

Permanently.

Permanently.

And that is the crux of why HIV is incurable.

That integrated pro -virus can just sit there, dormant, hidden.

The latent reservoir.

Precisely.

Especially in long -lived memory T cells, even with powerful antiretroviral drugs that stop active replication, that reservoir remains.

So the treatment goal isn't cure, but suppression.

It's to suppress the virus to undetectable levels, make it a manageable chronic condition.

But if you stop the drugs, the virus can reactivate from that latent reservoir.

Eradication is currently impossible.

That integration mechanism is just incredibly effective for the virus.

Let's contrast that with latency in herpes simplex viruses, HSV1 and 2, cold sores, general herpes.

Also direct contact, also establish latency in neurons, similar to VZV, they retreat to nerve ganglia.

But you mentioned a specific molecular trick they use to stay safe in those neurons.

Why don't the neurons just self -destruct when they realize they're infected?

Ah, yes.

The virus actively prevents that.

It produces something called the Latency Associated Transcript, or LAT.

And a piece of that, a microRNA called MIR -LAT.

And what does that do?

It specifically interferes with the host cell's apoptosis pathways, the programmed cell death signals.

So the virus basically tells the neuron, nope, you're staying alive because I need you as my hideout.

That's pretty much it.

It forces the cell to survive, ensuring its own long -term shelter for reactivation later.

Incredible manipulation.

Okay, moving to viral hepatitis via direct contact blood body fluids.

Hepatitis B, HBV.

This one's unusual, right?

A DNA virus that uses reverse transcription.

Yeah, it's a reverse transcribing DNA virus, very unique replication cycle.

But structurally, it has this clever immune evasion tactic.

Infected people produce the complete infectious virus particle called the Dane particle.

But they also shed massive amounts of incomplete, non -infectious particles.

Little spheres and filaments made of just the surface protein, HBSAG.

And these act as decoys.

Exactly.

They vastly outnumber the real virus particles.

They basically soak up the host's antibodies, acting like flares or chaff, protecting the actual infectious virions from being neutralized.

Clever.

And that HBSAG is what we test for and use in the vaccine.

Correct.

Screening is based on detecting HBSAG, and the vaccine is made from recombinant HBSAG protein.

Then there's hepatitis C, HCV, an RNA virus, Flaviridae family.

This used to be a huge problem for liver transplants.

The leading cause for a long time.

Chronic HCV infection often led to cirrhosis and liver cancer.

But the treatment story here is remarkable.

Truly one of modern medicine's biggest success stories.

Direct acting antiviral drugs, often combinations like Sophosbuvirulata -Pasphere, taken for just 8 or 12 weeks, achieve a cure.

A complete cure in over 95 % of patients.

It went from a chronic, often progressive disease, to something curable relatively quickly.

Amazing progress.

We should also mention hepatitis D, HDV.

The odd one out.

Yeah, HDV is fascinating.

It's a satellite virus.

It can only replicate if the person is also infected with hepatitis B.

Why is that?

It needs HPV's surface antigen, HBSAG, to provide the envelope, the outer coat, for its own new virus particles.

It's completely dependent on HPV.

Wow.

Okay, last one to direct contact.

Human papillomavirus HPV causes warts, non -enveloped DNA virus.

Yes, but the critical aspect here is cancer, oncogenesis.

Certain strains are high risk.

Very much so.

Strains like HPV 16 and 18 are the main drivers of cervical cancer.

Others, like 6 and 11, cause most genital warts.

Together, these few strains account for the vast majority of HPV -related diseases.

Which is why the vaccine is so important.

Absolutely.

The quadriaglin and now 9 -valent vaccines protect against those key high -risk and wart -causing strains, a major public health tool against cancer.

Okay, we've gone through viruses with RNA genomes, DNA genomes, reverse transcription, but now we have to tackle something completely different.

Prions.

The protein -only pathogens.

A total paradise.

DNA, no RNA, just protein, causing transmissible spongiform encephalopathies, or TSEs.

Right.

The idea is that a normal host protein, found mostly in the brain, called PRPC,

somehow misfolds into an abnormal shape, PRPSC.

And that bad shape is infectious.

Incredibly so.

The PRPSC protein acts like a template, or maybe a catalyst.

When it encounters a normal PRPC protein, it induces the normal one to misfold into the abnormal PRPSC shape.

It's like a chain reaction of misfolding.

Exactly, a cascade.

These misfolded proteins aggregate, forming plaques, and they are resistant to being broken down.

This leads to neuronal death and that characteristic spongy appearance of the brain tissues.

Spongiform degeneration, leading to dementia, loss of motor control.

Devastating.

And fatal.

And these diseases can be acquired, like variant Creutzfeldt -Eckob disease, VCJD, from eating beef contaminated with bovine spongiform encephalopathy, mad cow disease.

Like Kuru.

Kuru, famously studied in Papua New Guinea, spread through ritualistic cannibalism, specifically consuming infected brain tissue.

But there are also sporadic forms of CJD that just seem to arise spontaneously, and familial forms linked to mutations in the PRPC gene.

The fact that a protein alone can transmit disease,

it really does challenge the fundamentals.

It absolutely does.

No genetic material needed for transmission still blows my mind, really.

That protein paradox is a powerful place to end our run through these pathogen mechanisms.

I think so.

We've covered a lot of ground.

From, you know, the subtle versus drastic genetic changes in influenza drift and shift.

To that bizarre antibody -dependent enhancement in dengue, where immunity backfires.

Right.

And the deep complexities of retroviral latency with HIV, hiding in our own DNA, versus the active survival mechanisms of herpes viruses in neurons using things like LAT.

So thinking about all this, we talked about viruses like VZV, HSV, HIV -establishing latency, hiding out, often asymptomatically, for years, decades even.

Yeah.

And others we didn't even get into detail on, like Zika or EBV, can often cause asymptomatic infections, but still lead to immunity or sometimes severe complications later.

So here's the final thought then.

If we connect all these dots, how much of the immunity we see in the population today might be built on past infections that nobody even noticed they had.

That's a really interesting point, this hidden asymptomatic viral burden.

What does it mean for how we predict future epidemics?

What unseen reservoirs are out there, potentially shaping the emergence of the next big threat?

Something definitely worth mulling over.