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  • Intro 0:00
  • Virus Basics 0:04
    • Non-Living Structures have the Potential to Harm Life on Earth
    • Made of Nucleic Acids Wrapped in a Protein Coat
    • 5 to 300 nm Wide
  • Virus Structure 4:29
    • Icosahedral
    • Spherical
    • Bacteriophage
    • Helical
  • How Do They Invade Cells? 11:24
    • Viruses Can Fool Cells to Let Them In
    • Viruses Use the Organelles of the Host
    • Viruses are Host Specific
  • Viral Cycle 16:18
    • Lytic Cycle
    • Lysogenic Cycle
    • Connection Between Lytic/ Lysogenic
  • Retroviruses 30:04
    • Process is Backwards
    • Reverse Transcriptase
    • Example
  • HIV/ AIDS 32:38
    • Human Immunodeficiency Virus
    • Acquired Immunodeficiency Syndrome
  • Smallpox: A Brief History 37:06
    • One of the Most Harmful Viral Diseases in Human History
    • History
  • Prions 41:32
    • Infectious Proteins That Damage the Nervous System
    • Cause Transmittable Spongiform Encephalopathies
    • No Known Cure

Transcription: Viruses

Hi, welcome back to, this is the lesson on viruses.0000

Some virus basics, what are these things?0005

We are not talking about the computer kind.0008

The computer kind is named after the actual biological virus.0010

They are non-living structures that have the potential to harm every other life form on earth.0014

I do not want you to think that one single virus, like one type of virus can infect everything on its own.0020

That is not what I mean, I mean that any life form you can think of, bacteria, fungus, amoebas, plants, any animal, 0027

there is a virus that can invade its body and potentially kill it.0036

That is amazing to think about, the viruses being so tiny can be so harmful.0041

They are considered non-living, why?0047

For talking about them in biology and they are able to invade and infect living beings, what makes them not alive?0051

Couple of main reasons, they cannot metabolize nutrients.0058

When we think of every single other life form, it is made of cells.0062

A virus technically is not made of cells and because of that, because it does not have does cellular parts like ribosomes0067

and other particular enzymes to actually break down nutrients to get energy, we cannot consider it technically alive.0074

It does not have the ability to do that.0083

It actually takes advantage of the hosts ability to do that, so that it can make more copies of itself.0085

Along with that, it cannot replicate without a host.0091

There actually are some living parasites that this is true of as well.0095

There are some parasites that have to be in an animal body to reproduce and then0100

they leave that animal body, invade another animal, and so on.0105

These two reasons combined are major part of why viruses are not in the taxonomic groups.0108

Typically they are not considered alive.0115

Some scientists do not necessarily agree with that.0119

As time goes on and they are studied more, perhaps the definition of what life is maybe adjusted to include viruses.0122

But currently, when you look in most biology textbooks or resources, viruses are not considered living.0129

What are they structurally?0135

They are made of nucleic acids, DNA and RNA, wrapped in a protein coat and that is known as a capsid.0137

Here you are seeing a computer generated image of a virus.0143

All of this blue green stuff that is the capsid, that is the protein and these are known as antigens, 0148

little protein unit sticking out of it.0156

Inside, you have either DNA or RNA.0158

Viruses are either DNA viruses or RNA viruses.0161

That is the bare minimum, to be a virus is this protein coat or wrapping around genetic information.0165

Viruses can have other tools, they can have an additional membrane on top of their capsid.0172

They sometimes have little enzymes inside of them.0177

But you do not see cellular parts, you do not see ribosomes, you do not see organelles.0180

They are usually very tiny compared to even a bacterial cell.0187

Most viruses are 5 to 300 nm wide.0193

A nanometer is a billionth of a meter.0198

Billionth, that means in 1 m there is 1 billion nm.0207

Typically with measuring cells that are alive and much bigger, you are going to use the micrometer.0214

A micrometer is a millionth of a meter.0221

In 1 nm there are a thousand of these micrometers.0224

That is a very huge size difference.0230

That is a distinction to make because many viruses, if you lay them end to end to end,0232

a thousand of them across can fit in one cell.0237

Very huge difference in the ratio sizes from the average cell.0243

And the virus that would invade that cell and kick its butt.0248

Speaking of how many viruses you can fit in a space, I have seen this amazing fact that,0251

in the average period, at the end of the sentence, you can fit 10,000 cold viruses, side by side in that period.0257

So incredibly tiny.0266

Virus structures, there is two different kinds to keep in mind.0270

This is the first slide on virus structure.0274

There are a few major classes that occur in nature.0276

You could have viruses beside this, I’m going to give you the 4 main ones.0279

Here is an actual micrograph with an electron microscope, of what these look like.0282

Here is a computer generated image of what is called an adenovirus, 0287

these are typically causing respiratory infections like the cold.0290

Common cold is an adenovirus.0295

It is Icosahedral, in terms of the geometry of this.0297

It is a lot of amino acids side by side, in triangular shapes.0300

Those triangles all together can make a slightly spherical form.0305

But because of the triangle points, it does have these little pointy edges.0310

You have got what are called antigens here, these little protein units sticking out from there.0315

These little units sticking off in the adenovirus are kind of like the key that fits into the lock of the whole cell that it is going to invade.0321

I will tell you more about that, later on in this lesson.0329

Another virus, spherical, truly spherical, you do not see those little pointy edges.0334

You still do have these projections coming out from the outside.0339

A very common spherical shaped virus is influenza.0344

This causes the flu, there is influenza A, B.0348

There is a lot of different strains of the flu virus.0352

You can see it in action here.0355

You actually have a lot going on in this flu virus.0359

The capsid is this inner part here, that is the protein layer, the bullish purple part.0363

In this part that is a little bit more magenta is a lipid envelope on the outside, 0371

just another structural tool that this virus has.0376

A couple more virus structures, bacteriophages.0382

Bacteriophages strictly invade bacteria.0385

They can get inside of your body but they are not going to be able to invade your bodily cells.0390

Here is a micrograph of what they look like, pretty interesting.0395

And then, this is a computer generated image.0399

You can see something that is hard to tell in this micrograph, these things called tail fibers.0403

These things that look like legs are actually called tail fibers.0408

Oftentimes, this part is called the tail, I have also heard it called a neck or a mid piece.0418

Here is the head or the capsid.0424

Inside you got genetic material of DNA and RNA.0429

Oftentimes it is DNA with bacteriophages.0432

This invades bacterial cells like E coli, salmonella.0435

It is specialized to invade a bacterium, take advantage of its parts inside, make more copies of itself and bust out of there.0439

The way that it actually gets inside of the cells are very interesting.0447

Normally, you do not see all of this protein stuff that looks like an alien spaceship.0450

You do not see that going inside of the host bacterium.0456

They will just squirt their genetic information in.0459

Once that is inside, typically the bacterium is toast, unless it has restriction enzymes 0462

which you will hear more about in other lessons in this course.0468

As long as it gets in and this is how it does, I’m going to do a little impersonation for you.0472

Let us say that my tablet here is the cell wall.0477

The outside surface of the cell wall, the host I want to invade.0481

I’m the bacteriophage, my arms here are the tail fibers.0484

I land here and they can do a stapling action.0488

That stapling action, kind of, in one instant will not only allow the squirting of the DNA into the bacterium.0492

The way that it gets inside is there is a little bunch of enzymes here that cause kind of destruction, I’m making little splash here.0503

You are making a whole in the cell wall, those enzymes is almost like strapping C4 on the wall and blowing up.0517

It gets a big hole in the cell wall so that it can actually get the DNA inside of it, pretty cool.0528

Another kind of viral shape, helical.0537

You can see that the arrangement of the amino acids here is in that spiral shape that we see with DNA.0539

It kind of looks like a rod.0546

This is up close, you can see there is 18 nm diameter here.0548

We are looking actually at a very small part of what looks like a long rod.0554

This whole thing is the virus, these are three viruses here, 300 nm long each.0559

When we zoom into little part, you can see structurally a little bit more detail.0564

Here is the RNA that is inside of the hollowed out center of this helical virus.0569

TMV that stands for tobacco mosaic virus, this invades plants.0575

Not just the tobacco plant but that is where it was discovered.0590

This tobacco mosaic virus TMV, can invade tobacco plants and relatives of that plant.0593

Here is an amazing story that I have heard about TMV.0600

You could potentially have this virus in tobacco leaves, before a farmer whose growing them even knows it.0602

Before the virus has spread throughout the leaf to cause damage that can be seen.0609

The farmer and his workers, they pick the leaves, dry them out, shred them up.0614

Sell them to cigarette companies so that they can put them in their cigarettes .0620

After the drying out of the leaf and the shredding of the leaf, the virus is still intact inside those dried out dead cells.0627

The virus can remain there.0635

The amazing thing is the shredded up tobacco can end up in cigarettes.0637

A person could take a cigarette out of one of those packages, light it up, inhale it 0641

and then it had the virus actually come into their lungs through that inhaled vapor.0647

They could exhale it on another tobacco plant, a relative of that and the virus could get into the new plants.0654

It is not going to invade the lung tissue of the person who is smoking because TMC specifically is a plant virus.0663

It cannot actually invade animal cells, the proteins are too different.0670

But what is amazing is that it can survive the drying out, the vaporization thing.0675

They are so tiny, they just can hang with it.0680

How do they actually invade cells?0685

Viruses have the ability, thanks to their genetic code, to fool cells into letting them in.0687

Supposedly viruses have been around, ever since cells have been around.0692

Cells have evolved, as viruses have evolved.0695

There are genes in the viruses that allow them to mimic protein structure that gets them to fool the host cell.0700

Like I said earlier with that key and lock analogy, it is a good way to put it because 0709

they slip through the outer membrane or cell wall of the cell and get in there.0714

That is how they actually do their dirty work.0720

Either the whole virus enters, it can actually completely pass through the plasma membrane 0722

or just the DNA and RNA is inserted, like in the case of a bacteriophage.0728

Sometimes with viruses, more will go in besides just the genetic info like with RNA viruses called retroviruses.0734

There is also a very important enzyme that the virus has to get inside of the cell.0742

More on that, a little bit later in this lesson.0746

Viruses usually use the organelles of the host to make more copies of themselves.0749

That is how they actually can be successful and propagate, meaning make more parts, make more viruses.0753

Once they have used the cell, used and abused it, they typically leave, killing the cell.0762

And they move on to another host and do the same thing all over again.0769

It is almost like they are little organic machines.0773

Viruses are host specific, I have hinted at this earlier in the lesson.0776

Plant viruses do not invade animals.0780

When we look at in animal viruses vice versa would not invade plants.0783

When we look at different animals, you also have host specificity based on the type of animal.0788

Occasionally, it can hop around to different species.0795

Let me give you some examples.0798

Since the proper membrane, proteins must be accessed.0801

Viruses typically only can enter one type of host.0804

Viruses that infect us humans, most of the time they are just infecting humans.0808

It would not be that hard for them to hop to a chimpanzee or a gorilla because 0814

they are very closely related to us compared to other animals.0818

To go from us to a sea jelly which is not closely related, that is a big jump.0823

The proteins are way too different.0828

Some exceptions to this can exist.0831

You have heard about some of these in the news, potentially.0833

Avian flu also known as bird flu, affecting humans.0836

Several years ago, this was a major epidemic.0840

Avian flu typically just infects birds, it mutated just enough to affect the lung tissue of humans.0844

It could happen in reverse, a human virus can potentially mutate to infect birds.0857

It is possible.0862

You may have heard of swine flu, the actual better term for what that was is H1N1.0864

A lot of viruses are being presented or named with a letter and number combination, 0872

to keep them straight into identifying one vs. the other.0878

H1N1, supposedly that started in a population of pigs and got into human population, it affected us.0882

It is still around but it certainly has calmed down, in terms of how widespread it is.0891

HIV origin, where did HIV originally come from?0898

It did not just randomly pop up in the late 70’s or early 80’s.0902

That is when it did become widespread enough as an epidemic to get a lot of press and media,0906

and people started hearing about it, and it was named and so on.0911

Then, we figure out more about how it spread from person to person.0916

I have more about HIV later on in this lesson.0919

In terms of what it actually started, there are guesses, in terms of what decade it originally entered a human.0923

There is a good chance that it came from a chimpanzee.0930

One of the theories I have heard is that, someone in Africa was potentially butchering a chimp for food.0933

If that chimp had HIV in its bloodstream, some of that blood spattering on the person who is butchering them.0941

If they had a cut, it is possible that the virus could have been transmitted into them from the chimpanzee.0949

If it mutated just a little bit, it can hop from the genome of the chimp to ours, because that is 98.5% the same.0959

It is not that farfetched to think of that, possibly being the origin of the HIV.0969

It could come from another primate species.0975

Viral cycles, viruses have a way that they reproduce.0980

Just like when we look at cells with mitosis or you look at binary fission of bacteria.0984

Viruses have this pattern that is predictable, in terms of how they make more viruses.0989

The first cycle I want to tell you about is the lytic cycle.0994

You will see in the next slide, I will also tell you about the lysogenic cycle which is a longer word.0999

The lytic cycle in a sense, is a short term assault.1004

The way that I remember that is lytic is a shorter word and it is more of the short term of how it gets inside of the host cell,1008

takes advantage of its parts, and then quickly leaves and destroys it.1020

A lysogenic can be much more drawn out, as you will see in the next slide. 1024

A virus invades a host cell in the lytic cycle, makes copies of itself, and leaves the host, and leaves it dead.1028

Once it out of there, the cell, does not much have you can do, the contents of the cell leak out, it is done.1036

There are five main steps, attachment.1043

I will draw this for you in a couple of slides.1045

Attachment, the virus physically makes contact with the host.1049

Sometimes, it just squirts in its DNA or RNA.1052

Sometimes the whole cell actually goes in and that would be entry.1055

Entry of the genetic info or the whole virus.1059

Replication, the DNA inside the virus is going to be replicated with enzymes like DNA polymerase which were inside the host.1063

It uses the host’s enzymes to do that.1071

DNA is still phosphate sugar base, when you look at a virus.1073

It still has that 3 prime to 5 prime designation and 5 prime to 3 prime thing going on.1077

It is also going to use that replicated DNA to make RNA.1082

That RNA will be read at the ribosomes of the host.1088

It is going to make all kinds of viral parts, for talking a bacteriophage, 1090

it is going to make the tail, the tail fibers, the capsids, all those parts.1095

And then finally, those parts are assembled, they are put together with the new pieces of DNA inside the capsids.1100

It will be hundreds, sometimes thousands of virus from a single host.1107

And then finally, the release, they have a way of busting out of there.1112

Oftentimes, I just call the release stage busting out.1117

This is actually a micrograph of viruses in the process of docking on host,1122

which you can see here is the membrane and getting inside of there.1126

That is the lytic cycle.1132

The next viral cycle is lysogenic.1134

Like I mentioned earlier, this is that longer name.1142

It is typically more of a drawn out cycle, you will see why.1144

A virus invades a host cell here, inserts its genetic info into the host genome, that is different than lytic.1150

This time the DNA from the virus sneaks into the chromosome of the host.1156

It can lie dormant, for sometimes years, it really depends on the virus, it depends on the chemical factors.1163

I will give you an example in a second, with this particular picture which I know is not pleasant to look at.1171

The steps, if a virus was into a host and starts with the lysogenic cycle, the first two steps are identical to lytic.1176

First, the virus attaches to the surface and enters.1184

Sometimes, just like the genetic info or the whole virus enters.1188

Step three, it is different, it does not go to replication this time.1192

It goes to a provirus state.1196

I will show you on the next slide with a drawing of what provirus is.1199

It is basically the virus inserting its genetic info inside of the hosts genome.1203

From that point in time, the host unbeknownst to it, it just does what it normally does.1211

It eventually divides, copies all its DNA which means it is also copying the provirus, passes that on to daughter cells.1218

The daughters cells eventually divide.1226

This keeps happening where the viral DNA, which has been sucked into the host, keeps getting copied and passed on.1228

You can have potentially millions and millions of cells, as a result of these cell divisions.1235

Eventually, someday, it could be hours later, but sometimes it could be years later,1243

the provirus gets released out of these daughters cells, and then you go back to lytic.1249

Then it goes back to that step three of lytic where, now that the viral DNA has been released out,1255

copy it, make RNA from it, make viral parts, etc.1261

And then eventually, it is going to conclude with the virus wreaking habit and destroying those cells.1265

This particular image here, this is shingles.1273

I do not mean shingles on a roof, this is actually the chickenpox virus back with a vengeance, that is what shingles is.1281

I have had a chickenpox virus. I had it when I was about 6 or 7 years old.1289

I got over it, the virus went away, supposedly.1296

That is not necessarily the case, what could have happened with this chickenpox virus is 1302

my immune system fought it off, my skin healed.1306

I no longer have that little pox, little red marks that are itchy.1309

What could happen is, surviving viruses, viruses that remain, 1314

did not get destroyed by immune system actually went into this lysogenic cycle in some of the cells of my body.1319

And then years later, sometimes people who have chickenpox, approximately,1328

I have heard 1/3 of people with chickenpox will get shingles.1333

Where some chemical factors maybe it is stress, who knows, there are various theories about what brings it out.1336

Chemical factors in the body bring this chickenpox virus back and it comes back with a vengeance.1343

These rashes that tend to be concentrated on the torso, the front and the back, 1350

they really burn and they hurt way more than the chickenpox virus.1355

That shingles, the lysogenic virus is what connects the original chickenpox virus to this virus,1361

that now has gone back in the lytic cycle and wreaks havoc on the person's body.1370

Maybe one day, in my 50’s or 60’s,I can get shingles, I hope I do not.1375

Here is the connection between the lytic and lysogenic.1382

I want to give you a little play by play here.1384

The lytic I’m going to use blue arrows, for the lysogenic I’m going to use purple.1391

Step 1, this is going to be the first step of lytic.1401

Remember, the first step is attachment.1410

That is a bacteriophage, this is also known just as phage.1421

Sometimes you will hear T4T4 phage.1426

This is definitely a bacterium that I’m drawing, here is the host cell,1429

that bacteriophage that is docked in the outside, that is attachment.1433

Let us say that the host genome is yellow, that is that circular single chromosome that the bacterium has.1438

Onto step 2, there is the host, once again alien spaceship, that is actually a bacteriophage.1452

Here is where you have the viral DNA going in.1468

This is that entry that I told you about before.1473

After entry, we have step 3, this is replication.1483

This just remains here, this some kind of shell of a bacteriophage, protenacious shell, it is just there.1495

Eventually, it could just break down and dissolve into pieces.1504

It is useless at this point because now the really meaningful part that viral DNA has gone inside,1511

you are going to make lots of copies of that.1518

You are going to use the enzymes of this bacterium, RNA polymerase to make RNA from that DNA.1521

You are going to make protein parts.1529

Here are a bunch of capsids, tail fibers, the tails.1533

Not enough room in this drawing to write it.1541

But there is enough room in the actual host cell to make hundreds, if not a thousand copies of this bacteriophage and all its parts.1543

That is replication, next is assembly.1552

The assembly is, you are going to see what look like the little offspring of that virus that docked on the outside.1571

And more of course, but I do not have enough rom.1587

They have that little DNA in there, they are ready to go.1590

Finally step 5, busting out.1595

There, out of there, you get the picture.1617

You got there viral DNA.1624

We will say that the 4th one already left, it kind of head start.1627

Yes, this host cell will die in the process, too bad.1632

That is the lytic cycle.1637

Step 5 will eventually lead back to step 1, because these are all going to go other host cells nearby and do it all over again.1639

We are going to go to the purple arrows because there is a fork in the road here.1650

Once viral DNA enters, it actually could go this route.1654

This is going to be, we will call step 3.1660

I should do purple numbers for this one to differentiate.1663

This is that provirus stage.1667

We have got the host genome and then inserted in there is the viral stuff.1676

Like I said before, phosphate sugar base, that is the structure of DNA of every organism on earth, including viral DNA.1685

It can easily just kind of slip in, cut and paste itself in there.1694

The host, oftentimes, will not even knew.1700

The next stage, I’m going to do two little arrows to represent the fact that it is cell division.1707

It divides and you get that viral DNA being passed on.1720

From there, eventually at some point, step 5, that provirus gets released from all these daughter cells, of course.1734

However many were made, at some point, each of them can get stimulated chemically to have the provirus exit.1768

Once again, you are going to go back to step 3, eventually.1777

Back in the lytic cycle, so that the virus can do its thing, actually make a lot more copies of itself.1782

This is a very strategic thing, this lysogenic part of the viral cycle allows it to remain dormant or remain hidden, 1790

kind of lay low until the time is right, and then back to the business.1798

Retroviruses, I have heard someone say like, oh my god that is so retro.1807

Maybe they are talking about a style of clothing or furniture, or something.1813

Retro means it was a style back in the day, and now you are bringing it back, you are trying to make it popular again.1819

Retro means backwards like it is blast from the past.1827

Retroviruses, this a good name for this kind of virus because they do something backwards compared to viruses that are not retro.1833

Typically, a DNA virus enters a host cell, makes RNA with host enzymes, and then vital parts are made.1842

That is tradition, that is what normal viruses do.1850

But a retrovirus does it backwards and that is the retro part of it.1852

It is an RNA virus, that is the genetic information that is naturally in virus, that makes DNA once the vile genome enters the host.1859

The virus has its own enzyme, reverse transcriptase.1868

What is transcription?1872

If you saw the RNA lesson, transcription is reading DNA to make RNA, that is transcription.1874

What will be the reverse of that, making DNA from RNA.1884

Reverse transcriptase is an enzyme, if its ends with ASE that is a big hint.1889

Reverse transcriptase is an enzyme that does reverse transcription.1894

It can actually take RNA and put together two sided DNA from it.1897

RNA will be the compliment to one side of the DNA, and then they just make the other side.1903

HIV is an example of a retrovirus.1907

Here is a computer generated image of what it looks like.1912

You have got RNA strands in here, that is the yellow stuff.1915

Reverse transcriptase, that is these two orange dots.1918

It is just part of the toolkit of this kind of virus.1923

The capsid, made of protein, is right here, there is a matrix around it.1926

With another envelope, a lipid membrane on the outside, with these little antigens pocking out from it.1931

You could see that these antigen parts have different designations or names.1938

This is retroviruses, part of the arsenal of the viruses, in terms of what they can do and how they do it.1946

HIV and AIDS, HIV stands for human immunodeficiency virus.1960

I have read about SIV, which is simian immunodeficiency virus which means it would impact other primates other than us.1967

HIV is our version of this but you can see, comparable or analogous viruses in other primates.1977

Like I told you earlier in the lesson, the theory is that HIV that came into us, originated in chimpanzees.1985

Transmission of HIV can happen through blood or sexually transmitted fluids.1992

The most common ways that people get it is having unprotected sex with somebody that has HIV.1997

Even having protected sex, sometimes contraceptive devices are not 100% effective.2004

Statistically, there is no contraceptive device that is 100% effective all the time.2010

Having contact with a person with HIV has risks to it, that means sexual contact.2017

It can also happen through blood, sharing tattoo needles which is rarely done.2025

But also sharing a needle for drugs can get it from one person to another.2031

Also, the other unfortunate part that is actually no fault of the recipient is, sometimes, rarely, 2037

it can happen through blood transfusions.2045

When I donated blood in the past, I know that the organizations that accept the blood, 2049

they do everything they can to make sure that, that person does not have HIV, they do lots of tests.2053

Occasionally, every once in a while, it happens where the donor actually end up giving it to the recipient.2060

Also, it is very common for people to know this know, I just want to state that, 2068

you cannot get HIV through saliva and not by touching someone.2076

Specially, in the 80’s and early 90’s, I recall people having some ridiculous view points about HIV.2082

It is because they were not educated about it, they really did not know otherwise.2089

But trust me, kissing someone with HIV is not going to pass it to you.2095

The virus infects helper T cells, this is a class of a white blood cells.2102

Helper T cells, the reason why when you get a cold, when you get pneumonia, 2106

when you get different viral infections, they last just a short while.2112

The cold will last a few days, the flu will last 1 or 2 weeks.2118

Why does it go away?2122

You can thank your white blood cells for kicking the viruses butt, and helper T cells are a big part of that.2123

Unfortunately, it infects those very cells that help you work against viruses and help you get better from an illness.2130

When someone contracts HIV and that retrovirus starts invading these T cells and going to more T cells over time,2141

eventually, it can get to the point where AIDS develops.2150

There are plenty of drugs now, these drug cocktails in combination that make it so that, 2154

it really prolongs the period of time for a person with HIV, whether or not they are going to get AIDS.2159

A good example would be Magic Johnson.2166

People these days who get HIV, there are lots of options, there are lots of ways that drugs can help it,2169

kind of not spread to the body as quick.2177

You can really make your life last a lot longer.2179

Eventually, what can happen is AIDS develops from HIV.2183

AIDS is acquired immune deficiency syndrome, that gets to the point where 2187

so many of your white blood cells have been compromised, 2192

that you can no longer fight off infections that typically are harmless, long term to the average healthy person.2196

AIDS results when you cannot fight off the cold, you cannot survive the flu, 2207

even chickenpox could kill someone with AIDS.2214

That patient can no longer fend off infections, that is the point where it can definitely be a fatal case.2218

Smallpox, this is one of the most harmful viral diseases in human history.2227

If you are thinking of the black plague, the bubonic plague, that is actually bacterial.2233

There are plenty of other harmful viruses.2238

HIV, yes that is a harmful virus, it is a very virulent virus meaning harmful.2242

Ebola is a very harmful virus.2247

In terms of the hundreds and hundreds of years that this existed, really thousands of years that2249

this existed in human history and how many lives it took, and how widespread it got, smallpox was a big deal.2257

This is actually a drawing from the 1500’s, depicting what happened to the Aztec people upon Europeans arriving there.2263

A little bit of history, 243 BCE stands for before the common era, smallpox epidemic in China, 2273

lots of people died from this epidemic at that point in time.2281

Flash forward, over a thousand years later and it is the common era also known as AD,2286

a hermit in China introduces a mild case in humans to build immunity.2291

This is a very early example of immunization, kind of having like a vaccination in a sense.2295

This particular individual actually helped a lot of Chinese people, 2302

to the point where they would not get sick from smallpox which is a great step.2308

1519, just approximately 500 years ago, invading conquistadors spread smallpox to Mexico, decimating the Aztecs.2314

Smallpox, at that time, was found in the old world, we can call the Americas the new world, at that point in time.2329

It was one of those diseases that people in Europe, those populations, over thousands of years have been exposed to.2339

Some people would survive it.2348

It is one of those cases where the Aztec people and people in the new world, this was a disease, 2351

this was a virus that was so foreign to them that it hit them so hard, and very quickly the population was decimated.2357

Sadly, it helped in Cortez and those European conquistadors taking over that population.2366

It really helped their invasion.2373

The Aztec peoples, the majority of them were affected by smallpox.2375

1796, almost 300 years later, Edward Jenner developed vaccines.2383

The first vaccination of smallpox, first official vaccine, that could be given to many people.2390

We can thank Jenner for that.2397

Flash forward to the 20th century, the Worldwide Health Organization adopts a plan to eradicate it.2400

The World Health Organization, they thought that this is something that we can actually get rid off.2408

We have the science, we have the medicine to be able to actually wipe this off the face of the earth.2413

A plan was made to do that, and amazingly within 5 years time, 1977, the last reported case was known in Somalia.2420

Since that time, we have not seen smallpox.2429

Now chances are smallpox is kept somewhere on earth, a sample of it is probably locked away in some lab, 2433

maybe in multiple places.2443

The hope is that humans will never be exposed to that ever again.2446

At this point, the human population, it is been awhile since we have been exposed.2451

If it was unleashed, that would be a form of bioterrorism.2458

It would be something that will really take the human immune system by surprise.2462

Let us hope that does not happen.2469

As time goes on, we get so many great advancements where we can come up with vaccines and defeat viruses.2470

Make it where we use our medicine and our abilities to conquer that virus, so that it does not harm us.2478

Viruses, they will continue to evolve and there are always be something that can harm our species.2486

Prions are infectious proteins that damage the nervous system overtime.2493

These are not technically viruses, but I’m grouping them into the virus lesson because they are smaller but still a protein thing.2497

Remember, viruses have that protein envelope with genetic information inside of it.2507

Prions, they do not have DNA and RNA, they are just misshaped proteins that cause transmittable spongiform encephalopathy.2512

What was that mean?2523

It means that these proteins, when they get into the nervous system, they cause degeneration of your neurons,2524

to the point where it does lead to death, eventually.2532

Mad cow disease in cattle, when these prions, a certain class of them, get inside of the brain of a cow.2536

They named it mad cow because before the cow dies, it starts acting kind of crazy.2545

It goes mad because of these prions causing that damage over time.2551

If mad cow disease gets into humans, it is known as Creutzfeldt-Jakob disease which is abbreviated as CJD.2556

The way that it gets from cows to humans, this is just a theory, 2567

there are many theories on how it happens but this is a widely accepted one.2571

If beef is being taken off of a dead cow, it is being butchered.2575

If the butcher who was actually getting that meat off of the carcass accidentally severs the brain casing 2581

or the spinal cord casing, the prion could get out of that region into the muscle tissue.2589

When that is sold to market, regardless of how much you cook the muscle tissue,2596

regardless of how well done you make that meat, the prion lasts, it will not go away.2600

If you eat that contaminated beef, it can end up inside of your nervous system.2606

There is no cure right now.2612

Scrapie is the version of this prion disorder in sheep.2615

Like I said, there is no known cure.2621

Once these infectious proteins are in your body, they cause gradual degeneration of tissue.2623

Think of it this way, over time, they cause normal proteins to mutate.2628

These prions bump into the proteins of your nervous tissue, it is this kind of negative domino effect.2632

Over time, it gets more widespread.2641

The brain tissue, if you were to look at an individual that has CJD or an animal that has this spongiform encephalopathy,2643

the brain tissue gradually atrophies over time because of this negative prion effect.2653

That is prions, thank you for watching