Human Diseases Caused by Viruses and Prions

1:06with a massive sobering photograph. They show the AIDS Memorial Quilt spread across the U .S. Capitol back in the 80s and 90s. And every single patch represents a life lost to HIV. That image really anchors everything else we study in infectious diseases because, you An HIV diagnosis was effectively terminal, but then the introduction of highly effective antiretroviral drug cocktails. The famous triple therapy. Exactly. That shifted it to a manageable chronic condition and that breakthrough completely restructured the pharmaceutical landscape.

1:41It forced science to decode the human immune system on this incredibly deep molecular level. It really taught us how rapidly a virus can emerge and just devastate a population. Lessons that unfortunately we had to apply all over again in 2020 with the emergence of SARS -CoV -2. Very painful lessons, yeah. Well, welcome to this special deep dive crafted for the last minute lecture series. Today we're taking a comprehensive journey through human diseases caused by viruses and prions directly following chapter 37.

2:10Our mission is to help you master this material for your microbiology exam by looking at basically how these tiny pathogens travel, how they invade and the intricate biological machinery they use to survive. We are going to unpack the actual mechanism, so you're completely ready for that test. Right. So to understand how just a strip of genetic material wrapped in a protein shell causes such massive disruption, we really have to start with the ones that travel through the air.

2:36Which actually presents a totally unique physical challenge for a virus because air itself doesn't support viral replication. Right. It can't just grow in a cloud. No, a virus cannot multiply in midair the way, say, bacteria might multiply in a puddle of stagnant water. So any airborne virus has to be physically launched from a living source. And in humans, I mean, that means it's propelled from the respiratory tract by coughing, sneezing, or, you know, even just talking.

3:02Yeah. Let's look at the first major airborne threat the book covers, chickenpox and shingles. Caused by the varicella zoster virus or VZV. Right, which is a double -stranded DNA herpes virus. So when a kid catches it, they get that classic full body of a circular rash, red fluid -filled blisters everywhere. But the really strange part is what happens after the kid recovers. The virus doesn't actually leave the body, right?

3:25No, it doesn't. It sets up what we call a latent infection. It basically hides out in the dorthal root ganglia of the nervous system as a circular piece of DNA called an episome. So it's just chilling in your nerves. Exactly. The viral DNA is maintained inside the nuclei of your cranial nerves and sensory neurons. At this stage, you literally cannot detect any active virus particles. The infection is completely dormant.

3:51So if the virus is just sitting there dormant, is it essentially acting like a seed buried in dry soil just waiting for a rainstorm? A seed waiting for a rainstorm is a perfect way to visualize latency. The rainstorm in this case is anything that compromises the host's immune system. Like getting older. Right. As we age, our cellular immunity naturally wanes. Or maybe the host undergoes an organ transplant, chemotherapy, or even just experiences really severe physiological stress.

4:17And that stress waters the seed. Exactly. The viral genome activates, it begins assembling new virions, and those virions migrate all the way down the sensory nerves. Creating a brand new infection decades later shingles. And because it travels down specific nerves, it produces this incredibly painful striped rash that wraps around just one side of the torso or face, literally following the exact path of the infected nerve. From a childhood cough to intense nerve pain 50 years later.

4:45It's a very dramatic life cycle. It really is. But moving to a more recent airborne threat, COVID -19, SARS -CoV -2 is an RNA virus. And if you look at figure 37 .3 under an electron microscope, it's this tiny sphere with a corona or halo of spike proteins. And those spikes are the critical mechanism of entry. I mean, the virus has a massive genome for an RNA virus, about 30 ,000 nucleotides.

5:10But those spike proteins are precision engineered to bind specifically to the human ACE2 receptor to gain entry into our respiratory cells. Which actually brings up a confusing point in the text about its genome. Standard neurology principles tell us that RNA viruses mutate constantly because they don't have error checking mechanisms. But the book says coronaviruses actually do have a proofreading enzyme, right? They do, yeah. An RNA dependent RNA polymerase that actually checks for errors.

5:35So since it has a relatively low mutation rate, why do we see so many distinct variants like Delta or Omicron emerge so quickly? I know that confused a lot of students. It really just comes down to sheer mathematical scale. The mutation rate per individual replication cycle is relatively low, thanks to that proofreading. But when you have hundreds of millions of people infected globally, and the virus is replicating trillions of times within each individual host.

6:01That low error rate still means millions of mistakes. Precisely. It translates into a massive number of random mutations. The virus is playing the evolutionary lottery so many times that it is mathematically guaranteed to hit the jackpot. Wow. Yeah. So mutations changed a single amino acid in the spike protein, allowing the virus to bind to that ACE2 receptor even tighter, or evade the antibodies we already had. And once it binds and enters the cell, the clinical progression of COVID -19 is really a tale of two phases.

6:30Phase one is driven by explosive viral replication. The patient gets that dry cough, fever, and sheds maximum amounts of the virus. But the severe life -threatening symptoms usually happen in phase two, which is actually driven by the host. Not everyone enters this second phase, but for those who do, the severe illness isn't caused directly by the virus replicating anymore. It's the immune system panicking. Yes, it's an exaggerated host inflammatory response.

6:57The immune system overproduces pro -inflammatory cytokines, specifically IL -1 and IL -6. This just floods the lungs with fluid and immune cells, which causes severe hypoxia. So they can't get enough oxygen. And it also triggers abnormal blood coagulation, right? Leading to micro thrombi, those tiny blood clots all over the body. Exactly. It's a massive systemic overreaction. Speaking of respiratory infections, we have to look at the flu because influenza completely breaks the biological rules for RNA viruses.

7:26The flu virus has two main envelope spikes, HA or hemagglutinin, which it uses to attach to the host cell, and NA, neuraminidase, which it uses to slice its way out of the cell after replicating. And its replication cycle is just truly unique. Most RNA viruses replicate out in the main compartment of the cell, the cytosol. Influenza completely ignores the cytosol and transports its RNA segments straight into the host cell's nucleus.

7:52So it's like a burglar who doesn't just break into your house but sneaks all the way into the hidden wall safe? That's a great analogy. And once it is inside the nucleus, it performs a process called cap snatching, which is an incredible survival mechanism. Oh, this part is fascinating. So for a virus to multiply, it needs the host's ribosomes to read its viral mRNA and build viral proteins.

8:13But human ribosomes only recognize mRNA that has a specific molecular identification tag called a 5' cap. And influenza doesn't have its own caps. Right. So a viral enzyme literally cuts the 5' caps off the host's newly synthesized mRNA transcripts and stitches them onto its own viral RNA. That's crazy. It literally steals the host's ID badge to get past cellular security. So that explains how it replicates. But for the exam, we also have to understand why we need a new flu shot every single year.

8:42And this involves the difference between antigenic drift and antigenic shift. Two very important terms. So drift refers to small seasonal point mutations. The RNA polymerase makes a tiny error, the spike protein changes just a little bit, and last year's immunity is slightly less effective. But shift, antigenic shift, is the mechanism that causes global pandemics. Yeah, and antigenic shift requires a biological mixing vessel, which is very often a pig.

9:10Influenza has a segmented RNA genome. It's broken up to linear pieces rather than just one continuous strand. So if a single cell in a pig's respiratory tract is simultaneously infected with a human flu virus and an avian flu virus, those individual RNA segments can get physically shuffled and repackaged together inside the host cell. Oh, wow. So when the pig coughs, it releases a completely novel recombinant virus with a combination of surface proteins the human immune system has literally never seen before.

9:37Which is terrifying, but biologically brilliant. Definitely. Now, there are a few other major airborne threats we should touch on briefly. Measles, which is known for being astonishingly contagious. Clinically, it often presents first with cluplic spots, those tiny blue -white specks inside the mouth that appear before the classic skin rash. Then there are rhinoviruses, causing the common cold. And there are over a hundred different strains, which is why we can never build lifelong immunity to it.

10:05And then RSV, respiratory syncytial virus, which the text highlights as being particularly dangerous for infants. You're right. And the danger of RSV comes from a specific envelope structure called the F -protein. When RSV infects respiratory cells, this F -protein causes the plasma membranes of neighboring host cells to just melt together. Wait, why would a virus want host cells to fuse together? What's the advantage there? Well, it allows the virus to spread from cell to cell without ever entering the extracellular space.

10:35Remember, your antibodies float in the extracellular fluid. By fusing the cells into a single, giant, multinucleated mass, which we call a syncytium, the virus can travel through the respiratory tract, completely hidden from the host's antibodies. That's incredibly stealthy. But unfortunately for the host, that massive tissue damage causes severe inflammation that physically blocks the tiny airways of an infant. OK, the last airborne virus we need to cover is smallpox, or variola virus.

11:04Because just like the flu broke the rules for RNA viruses, smallpox breaks the rules for DNA viruses. Under an electron microscope, figure 37 .8 shows it as this massive brick -shaped particle with a very distinct dumbbell -shaped core. Right. And normally DNA viruses, like the herpes viruses we discussed earlier, they have to traffic their DNA into the host cell's nucleus to use the host's DNA replication enzymes.

11:28Smallpox has a genome that is so huge, containing around 186 ,000 base pairs, that it actually codes for all of its own replication machinery. So it doesn't need the nucleus at all? Not at all. It replicates entirely out in the cytosol. Incredible. Alright, so moving on from the airborne category, let's look at viruses that cannot survive a trip through the open air. A sneeze just won't work for them.

11:49They require a biological taxi. Which brings us to Section 37 .2, the arthropod -borne diseases, or arboviruses. These are viruses transmitted by blood -sucking vectors, primarily mosquitoes. And the interesting thing is, the virus multiplies inside the insect without actually harming the mosquito. It just uses it solely as a delivery mechanism. Two major positive -strand RNA viruses in this category are carried by the Aedes mosquito Chikungunya and Zika.

12:19Now, Chikungunya is notorious for causing severe, lingering joint pain. But Zika is known for the devastating effects it has on fetal development, specifically causing microcephaly. And one fascinating detail the book mentions about Zika's evolution is a specific mutation that increased its viremia, which is the concentration of the virus and the human bloodstream. But why does a virus need to maintain such high levels in the blood, rather than just localizing specific tissues?

12:42It's all about the taxi. The virus relies entirely on the mosquito to travel to the next human host. A mosquito bite only extracts a tiny, tiny droplet of blood. If the viral concentration in your bloodstream is low, there's a really good chance that tiny droplet won't contain enough virions to infect the mosquito. So by mutating to cause high viremia, Zika ensures that any mosquito that bites an infected human gets a massive dose of the virus.

13:08It guarantees the mosquito becomes a carrier, keeping that transmission cycle totally unbroken. Makes perfect sense. Now, what if a virus lacks a tough envelope to survive the open air, and it doesn't have an insect vector to transport it? These pathogens require direct, intimate contact. Kissing, sexual contact, sharing body fluids, section 37 .3. This category includes HIV, which we mentioned at the start. HIV is a retrovirus that specifically targets the host's immune cells by binding to the CD4 receptor protein, but this category also revisits the herpes viridae family.

13:41Right, the ones that establish latency. We have HSV1, which typically causes cold sores, and HSV2, which usually causes genital herpes. To establish that lifelong infection, the viral capsid physically hijacks the host cell's internal highway system. It travels along cellular microtubule tracks directly to the nuclear pores and injects its DNA straight into the nucleus. It's just wild to picture that happening inside our cells. This family also includes cytomegalovirus, or CMV, and if you look at the micrographs in the text, it's actually a bit unsettling.

14:14It forces the cell to swell up, and the nucleus develops these massive dark inclusion bodies that look exactly like giant owl eyes staring back at you. Very distinct visually. And then Epstein -Barr virus, or EBV, is another herpes virus transmitted through direct contact. It causes infectious mononucleosis. If you look at a blood smear from an EBV patient, the infected lymphocytes are completely deformed. They lose their normal round shape and become stretched, indented structures known as downy cells.

14:40The direct contact category also includes the hepatotides, the liver invaders. There are several types of hepatitis viruses, but hepatitis B, or HBV, has a particularly brilliant evasion tactic. It's a complex, reverse transcribing DNA virus. But when researchers look at the blood serum of an HBV patient, they see three completely different structures floating around. Right. Figure 37 .20. Yeah. And only one of them, the Dane particle, is the actual infectious virus containing the DNA.

15:11The other two structures are just empty spheres and long filaments made entirely of viral surface protein. And the virus produces those empty spheres and filaments in massive, overwhelming quantities. Are they functioning like decoy flares, like when a fighter jet drops burning magnesium to distract a heat -seeking missile? That is the exact mechanism, yes. Those empty shells are composed of hepatitis B surface When the host's immune system generates antibodies to fight the virus, those antibodies bind to the millions of harmless decoy flares.

15:39Wow. So the immune system just exhausts its resources attacking empty shells. Exactly. Allowing the actual infectious Dane particles to slip through the chaos untouched and infect new liver cells. That is devious. Okay, our final direct contact pathogen is HPV, human papillomavirus. It's a non -enveloped DNA virus responsible for warts, ranging from common plantar warts on your feet to genital warts. Importantly, several strains are highly oncogenic. But how exactly does a wart -causing virus lead to cancer?

16:13The oncogenic strains of HPV produce specific viral proteins that bind to and destroy the host cell's tumor suppressor proteins, primarily a protein called P53. And P53 is important because... Normally it acts as a cellular break. It stops a cell with damaged DNA from dividing. When HPV destroy those breaks, the host cell begins dividing uncontrollably, which is the foundational mechanism of cancer. Which is why the development of the HPV vaccine represents such a monumental public health achievement.

16:40We're literally vaccinating against a cancer -causing virus. Absolutely. Okay, let's shift from direct physical contact to the things we consume, food and waterborne viruses. To survive being eaten or drunk, these viruses must endure an incredibly hostile environment. The human stomach is essentially a vat of hydrochloric acid. Pathogens traveling via the fecal -oral route have to pass through this highly acidic environment to reach the intestines where they actually replicate.

17:07Viral gastroenteritis is frequently caused by rotavirus, which gets its name because its structure looks like a tiny wheel under the microscope. But the most historically significant virus in this category is poliovirus. Polio is an enterovirus. And what is fascinating about its structure is what it lacks. It is a non -enveloped virus. Which sounds counterintuitive, right? Yeah. I mean, you might think a virus would want a protective lipid envelope before going into a vat of stomach acid.

17:33Like putting on a coat of armor? You'd think so, but actually, lipid envelopes are highly vulnerable to acid and detergents. When a virus relies on an envelope for its attachment proteins, destroying the envelope destroys the virus's ability to infect. Poliovirus is naked. It consists solely of a tough, densely packed protein capsid. So no lipid envelope to melt away. Exactly. That naked protein shell is incredibly stable at extremely low pH levels.

18:00So that lack of an envelope is exactly what allows polio to survive at a pH of 2 .0 in the stomach. It survives in contaminated drinking water, passes through the stomach completely untouched, and replicates in the gut. And while it normally just causes mild gastrointestinal symptoms, in a small percentage of cases, the virus enters the bloodstream and attacks the central nervous system, destroying motor neurons and causing devastating infantile paralysis.

18:26Which brings us to a category of viruses that didn't evolve to navigate the human stomach or the human respiratory tract at all. Zoonotic viruses. Section 37 .5. These are adapted to animal reservoirs. Right. They happily exist in bats, rodents, or insects. But occasionally, a human encounters that animal reservoir, and the virus spills over into a human host. And because these viruses aren't adapted to humans, the resulting diseases are often terrifyingly severe.

18:52Ebola is a classic example. It's a filovirus, meaning its structure isn't a neat sphere or brick. It looks like a long, tangled, microscopic thread. Then we have rabies, which is highly neurotropic, meaning it targets the nervous system. It has a very distinct fatalistic bullet shape. Both very severe zoonotic diseases. But there is another virus in this zoonotic section that seems like it belongs somewhere else based on what we read.

19:16Hantavirus. Hantavirus pulmonary syndrome is caused by inhaling aerosolized rodent urine or droppings. If it's inhaled, shouldn't we have covered it in the airborne category with influenza and COVID -19? I see why you'd think that. But the mechanism of transmission determines the category. The airborne category covers viruses that transmit from human to human through the air. You cough on me, I get sick. Oh, I see. Hantavirus does not spread person to person.

19:42If you sweep up a dusty cabin and inhale aerosolized rodent droppings, yes, you can develop a severe respiratory illness. But you cannot cough and give hantavirus to the person sitting next to you. The human is a dead -end host. So the transmission strictly requires the animal reservoir, making it fundamentally a zoonotic spillover event. Exactly. The exact same logic applies to rabies. It requires a bite or saliva transfer directly from a rabid animal.

20:07You don't catch rabies from another human. That distinction makes perfect sense for the exam. Okay, for our final category, we are going to throw out everything we've talked about regarding genomes. We've discussed single -stranded RNA, double -stranded DNA, positive strands, reverse -transcribing genomes. And now we stripped the genome away entirely. We are talking about prions. A prion possesses absolutely zero nucleic acid. No DNA, no RNA, no genetic blueprint of any kind.

20:36It is purely a transmissible, misfolded protein. And yet, it causes universally fatal neurodegenerative diseases like Creutzfeldt -Jakob disease or CJD. It completely breaks the foundational rules of infectious biology. If it doesn't have a genome, how can a protein possibly replicate to cause a progressing disease? Well, it doesn't replicate in the traditional sense. It converts. Your brain naturally produces a normal cellular prion protein. A disease -causing prion is the exact same protein sequence, but it is physically folded into the wrong 3D shape.

21:08When this rogue prion bumps into a normal protein in your brain, it physically forces the normal protein to unfold and refold into the rogue shape. Whoa. So one misfolded protein converts another, and those two convert two more, creating this cascading exponential chain reaction of misfolding. Yes, and these rogue proteins clump together, ultimately destroying the brain tissue and leaving it full of microscopic holes. That is terrifying, but also amazing.

21:35It is a really sobering reminder that there is always more to learn. Biology rarely adheres to the strict boundaries we try to draw around it. Let's bring this journey full circle to help you lock in these concepts for your exam. We learned that smallpox, a massive DNA virus, was completely eradicated from the globe. It was possible because its DNA genome was very stable, didn't mutate rapidly, and the symptoms were incredibly obvious.

22:01Public health workers could easily identify and isolate cases. But as you close your textbook today, mull over this. Considering how easily RNA viruses like the flu -swapped genetic segments in pigs, or how zoonotic viruses hide silently in vast populations of bats and rodents, will humanity ever be able to eradicate another virus the way we did smallpox? It is a massive challenge waiting for the next generation of microbiologists.

22:23The interplay between viral mutation and human public health is just an endless arms race. It really is. Well, thank you so much for joining us on this deep dive. On behalf of the entire last minute lecture team, we wish you the absolute best of luck with your microbiology exam. Keep questioning, keep exploring, and we'll see you next time.