Hello.
I am David Baltimore.
I am a professor at the California Institute of Technology -- Caltech.
And I'm here to talk about viruses.
I've worked on viruses, now, for 50+ years.
And I find them fascinating.
And I'll concentrate on HIV because it's the virus most people know.
And of course it's the scourge of the 20th and 21st century.
So, I call viruses a separate kingdom of the living world.
They're not really a kingdom, but they are so different from any other organism that
they should be considered very separate.
They're extremely tiny.
They can only duplicate themselves by penetrating into living cells and diverting the cells' activities
towards the ends that are dictated by the genetics of the virus.
And thus they are obligate cellular parasites.
Viruses can only exist once cells... could only exist once cells have evolved.
They can have either RNA or DNA as their genetic material; the rest of the biological world
uses DNA.
And they can grow in all kinds of cells: in animal cells, in bacteria, in plants.
And the fact that so many of them are RNA viruses suggests that they're left over
from an RNA world, which was an intermediary in the evolution of the world we know today,
which is based on both DNA and RNA.
The central dogma of molecular biology hasn't changed in... since the 1950s, when it was
first enunciated by Francis Crick.
And that is that DNA can duplicate itself, it can be transcribed into RNA, and that RNA
can in turn be a template for making protein.
And protein is what makes life work.
All of our cells are built out of proteins.
All of the events of our lives are determined by proteins.
In 1970, I was working on a virus, and had the good fortune to make a discovery
that changed the central dogma slightly.
We discovered reverse transcription: RNA giving rise to DNA.
And that reverse transcription actually gives rise to a very large amount of the DNA
of us and of most higher organisms.
The discovery of reverse transcription filled out our understanding of the logic of
how viruses multiply.
And let me describe that logic by focusing on the most important product that's made
during a virus infection, which is messenger RNA.
Messenger RNA encodes protein.
And those proteins, now viral proteins, can take over a cell, can make more virus particles.
They are the essence of the virus infection process.
And so, the... the nucleic acid, the DNA or RNA of the virus, is holding the instructions
for making those proteins.
And the question is, how do those instructions get to making protein?
And there are really six ways that that can happen, depending on exactly how the instructions
are transmitted.
And those six ways distinguish one class of viruses from another class of viruses.
So, the most simple, straightforward ones are the double-stranded DNA viruses,
class I viruses.
Those simply make messenger RNA just like we make messenger RNA in our normal cells,
because we carry double-stranded DNA as our hereditary material.
Then there are single-stranded DNA viruses.
They have to become double strands -- those are the class II viruses -- and then they
can make messenger RNA.
And then they have to make, again, those single strands to be the genetic material of those viruses.
Then there are RNA viruses.
There are double-stranded RNA viruses, very similar to the double-stranded DNA viruses
except they're RNA.
And they make messenger RNA from their double-stranded RNA.
There are two kinds of single-stranded RNA viruses.
They can either carry the sense strand, which we call the plus strand, of genetic material,
in which case they can encode protein by just putting that into the cell.
Or they can, in the class V viruses, carry the minus strand, the complementary strand.
That's sense-less until it's copied into a plus strand.
And so it needs to be copied before it can make protein.
And the virus particle has to carry a polymerase to do that.
And then there's the reverse transcription process, in which an RNA is carried by the virus,
but it... it's copied into, first single-stranded DNA, then double-stranded DNA, and that then
is the template to make messenger RNA.
So, that gives us six different kinds of viruses.
I call them... it's called the Baltimore classification because we first enunciated it back in 1970,
and many textbooks use it as a way of organizing... thinking about viruses.
And... but it really isn't a classification.
It's a statement of how viruses fit their style of handling genetic information
into the needs of the central dogma.
Now, how many viruses are there?
There are an uncountable number of viruses.
Every animal in the animal kingdom has its own set of viruses; every bacterium has
its own set of viruses.
There are certainly a million viruses that are easily distinguishable, one from another,
and many estimates have been made that are much higher than that.
Because viruses can only grow inside cells, they have to get into a cell.
But once they get in there, they can very quickly take over the cell.
Some viruses, particularly of bacteria, have a life cycle of only 20 or 30 minutes.
And even mammalian viruses take only a few hours to make enormous numbers of copies of themselves.
They can continue to live only if they are passed on from one organism to another because,
being obligate parasites, they can't live out in the world on their own.
And so every virus is a story of transmission.
We know that transmission in humans can involve sneezing or touching a surface that
a sick person has touched.
But for cats, the transmission is different, for insects the transmission is different,
and that's part of the evolution of viruses.
They evolve to become part of the living style of their hosts.
If viruses are very particular to a species, like the human species, then we can actually
get rid of them because they have no reservoir outside of us.
And so if we can vaccinate the whole population of the world, then we can get rid of that virus,
because we make ourselves immune to it.
And we've done that with smallpox.
And we've almost done that with polio, and they believe that within a couple of years
polio will be eliminated from the Earth.
Now, viruses growing inside cells, for them to get out of cells, they've got to do something.
And they do one of two things, generally.
Either they bet... bud out from the surface of cells, and that's what this picture shows,
or they grow up to large numbers inside the cell and then they burst the cell open.
And that liberates virus particles from the cell interior into wherever it is, into the
blood system... bloodstream in the humans, for instance.
Viruses only made sense when molecular biology was born as a science, because they're
so small that the only really important thing that they have is their genetic instructions,
their DNA or their RNA.
They generally protect that in a coat, but the coat is... is largely inert, except for
helping them get into the next infected cell, or next cell that they're going to infect.
Some of them have a few other proteins that help initiate the infection.
And then, we're now learning about viruses that are very big and complicated -- surprising.
They're actually of the size of bacteria.
But what distinguishes them is that they require to be inside a cell to multiply, whereas bacteria
can multiply as free-living organisms.
The way we detect viruses is by what's called a plaque assay.
We have a lawn of susceptible cells.
They could be mammalian cells; they could be bacterial cells.
We put a few virus particles down.
Those virus particles chew up the lawn from a center where they land, and they form
a plaque.
And if you count the number of plaques, you know how many viruses were there.
And if you make... if you put down twice as many viruses, you get twice as many plaques,
so it's a highly quantitative test, and bacterial genetics was done using those tasks...
those techniques.
Here, we can see tiny plaques made by a kind of bacterial virus, or huge plaques made by
poliovirus plated on human cells.
A distinction which I find very important among viruses is the distinction between
equilibrium viruses and non-equilibrium viruses.
Equilibrium viruses have... are viruses that have been, for a very long time, parasites
of a particular species.
And so they're highly adapted to that species, to the lifestyle of that species, to the cells
of that species.
And what happens is that they evolve to actually be less lethal, because they don't want to
kill off their hosts.
If they killed off every member of the species, that virus would have... would die itself.
And so, they cause enough disease to help their spread, but they don't...
like causing sneezing or coughing... but they don't kill most people.
The common cold virus is a very good example of a... an equilibrium virus in humans.
Then, there are non-equilibrium viruses.
Non-equilibrium viruses are viruses that have jumped from one species to another.
They're not yet adapted to that new host.
They've only been with that new host for a while, even decades.
And they can be extremely lethal, because this is not a host they care about.
And so they haven't evolved to be non-lethal in that host.
They may in fact spread poorly, or they could spread very well.
They may be very efficient, very inefficient.
There's enormous variation.
But those are the viruses that cause us the biggest problems, because those viruses
can be lethal.
So, if we talk about equilibrium viruses, we talk about polio.
Polio, in its day, caused... was... was lethal to a very small fraction of the people
who were infected by it.
Most people didn't know they were infected -- 90%+.
A few people actually were paralyzed by it.
And a few people were actually killed by it.
And that's in the nature of an equilibrium virus, largely non-lethal,
although occasionally lethal.
Smallpox actually was... is... is a virus that's lethal to a fair number of people,
and you really want to get it when you're young and get over it.
And that was the nature of vaccination against smallpox in the very early days.
The common cold virus we mentioned.
Measles virus, another one, actually causes quite severe infections to a few people.
Most people get over it after being in bed for a couple of days.
Herpes viruses are all around us and are... keep infecting people, but are generally non-lethal.
Now, non-equilibrium viruses.
The most famous one of those is influenza virus.
It's a natural virus of birds.
In birds, it's generally not very lethal.
It comes to humans and it's lethal to older people and very young people.
And it's really a serious disease for adults to get.
And that's why we try to vaccinate against it, although our vaccines are nowhere near
as good as we'd like them to be.
HIV is a virus that recently came to us from chimpanzees, but in fact it's not even
a natural virus of chimpanzees.
It seems to be a natural virus of monkeys.
And they gave it to chimpanzees, which in turn gave it to us.
SARS is a virus, probably, from bats.
Ebola is a virus almost certainly from bats.
Hantaan was from rodents.
Those have been very lethal to humans, but generally in contained areas, because they...
they haven't evolved to be able to spread very effectively.
Now, let me turn to HIV, because it's a virus that we've heard so much about, and that
has been such a devastating virus to human beings.
In... in the last few decades, the spread of the human immunodeficiency virus
throughout the world has been a constant source of fear and news.
It came into our midst really in the early 1980s.
Before that, it was in human beings, but it was in very limited populations,
mainly in Africa.
Somewhere around the 1980s, it started spreading, coincident with travel.
It was taken on airplanes, in people, from its... its source in Africa into Europe,
into the United States.
Making sense of... of HIV depended on having discovered the reverse transcriptase.
So, our work in 1970 actually set the base for finding HIV, because HIV... it uses
a reverse transcriptase and it was that enzyme that revealed the nature of that virus.
So, HIV is the classic non-equilibrium virus.
It's, as I say, endemic in African monkeys.
It got a foothold in the human populations maybe 90 years ago.
For a long time, it was found only in villages in Africa.
Then, as... as transportation became more and more commonly used to take people
from one continent to another, it seeded other continents and became a worldwide epidemic.
It grows in one of the key cell types of our immune system.
And so it actually knocks out our immune systems.
And that's what... why it's a lethal virus.
We die of some infection, not of HIV infection.
And with that, I've finished with this discussion of viruses and of HIV.