Hello and welcome to the world of viruses.
After listening to this lecture today, I hope
you'll be able to know when and how viruses
were discovered. I hope you'll understand
the defining features of viruses. You should
appreciate that viruses are everywhere and
outnumber cellular life. You will know that
there are good viruses, as well as bad viruses.
And finally, you'll be able to answer the
question: Are viruses alive?
We live and prosper in a cloud of viruses.
Viruses infect all living things on the planet.
We regularly eat, touch and breathe billions
of virus particles on a daily basis. As I
sit here today, I'm probably inhaling some
viruses and depending on what you're doing,
if you're eating something, you're probably
eating viruses as well. We simply can't escape
them. More amazingly perhaps, is that we carry
viral genomes as part of our genetic material.
That's right, I'm about 8% viral and so are
you. The number of viruses on the planet is
staggering. Let's just consider the number
of viruses that infect bacteria, these are
called bacteriophages. In the waters of the
world, it's estimated that there are over
10 to the 30th bacteriophages; this is a huge
number, bigger than Avogadro's number. So
let's try and put this into a form that maybe
it's easier to understand.
A phage particle weighs about 10 to the -15
grams, that's a femtogram. So if we multiply
that by the 10th to the 30th total bacteriophage
particles on earth, we get a number that is
staggering. That biomass exceeds the weight
of elephants by over a thousand fold. So this
is a virus that you can't even see, but there
are so many of them on the planet that they
weigh a 1000 times more than elephants. Alright
so let's look at it another way, maybe you
don't care too much about that way of looking
at it, let's say we take these 10 to the 30th
bacteriophages and we lined them up end to
end in a straight line. That line of 10 to
the 30th phages would stretch for 100 million
light years. That's a long time. That's farther
than the nearest galaxy into space. Again,
these are particles that you can't even see,
but the point is that there are so many of
them, that they weigh a lot and that they
can go long distances when you line them up
end to end.
One of the main things I'd like you to take
away today is that viruses are not just purveyors
of bad news. I think most people on the planet
think when they hear the word virus, they
hear infection or disease. I'd like you to
understand that viruses are mostly good.
For example there are more viruses in a liter
of coastal seawater than there are people
on earth. Just go to the ocean some time with
a liter and pick up some of that ocean water
and if you look at it, you're looking at more
viruses in that liter than people on earth.
And these two pie charts illustrate this.
If you simply look at biomass in the waters,
prokaryotes are a greater fraction of biomass
than protists or viruses. But if you look
at the abundance in terms of the number of
particles, on the right-hand pie chart in
green, you can see that viruses outnumber
everything else on the planet. There is just
more of them than anyone else.
Another interesting fact is that today we
are in the middle of a pandemic caused by
HIV, human immunodeficiency virus, the agent
of AIDS. Right now we know that there are
10 to the 16th HIV genomes on the planet right
at this very moment. What does that number mean?
It's a big number, not as big as 10 to the
30th, but it's still big. Well what it means
is that among those 10 to the 6th genomes,
there are already genomes resistant to every
antiviral we have today. We have over
30 different antivirals to treat infections
with HIV, but there is already resistance
out there to every one of them. What's more
astonishing is that there is also resistance
to any new antiviral we would ever develop,
that's how big 10 to the 16th is. Each of us is
infected with at least a dozen viruses at