00:00
Each of us is infected with
at least a dozen viruses at
any given time, probably more. When I talk
to audiences about viruses, one of the first
questions I ask is, please raise your hand
if you're infected or has had a virus infection
and only about 10% of the people answer. But
the fact is that we're all infected, always.
00:21
I'm infected now and so are you and there
is no getting around it. We have at least
a dozen herpes viruses in us, and on this
slide is a picture of a herpes virus. It's
a large DNA containing virus. And we acquire
these infections when we are quite young, and
we keep them for our whole lives. Herpes simplex
viruses, varicella zoster, human cytomegalovirus,
Epstein-Barr virus, and a variety of other
herpes viruses, they all infect us, they stay
with us. Once we're infected, it's for life
and we think that some of these viruses may
in fact be beneficial for us. It's true that
from time to time they do cause disease, but
the fact that they're present throughout our
life we think its a clue that they may be helping
us.
01:08
An interesting point to look at in terms of
how viruses help us is in our genome, in our
DNA. The human genome is about 3.2 billion
bases in length and we know its sequence.
01:21
It has been sequenced in many different individuals,
so we know how it's made up. And let's take
a look at what makes up our genome. A fraction
of our genome is made of repeated elements
called sines and lines, in fact the part of
our genome that codes for protein is very
small, as you can see by this slice of the
pie. There are intervening sequences which
don't encode proteins, which are a big fraction
of our genome. Miscellaneous unique sequences
and heterochromatin. There are duplications
of DNA. There are sequence repeats. And then
there are DNA transposons which are quite
interesting. These are DNAs that hop around.
02:02
But perhaps for our discussion, the most relevant
point are the LTR retrotransposons. These are
viral sequences that are part of our genetic
information. You can see they make up a good
fraction of the DNA, they are about 8% of
our genome. They are result of infections
that happen many, many years ago, millions
of years ago and have remained with us ever
since. And in fact some of these genes encode
proteins that help us. So a virus gave us
a gene many years ago that is useful to us
today, so we're really truly viral.
02:40
All of these viruses that we inhale and eat
and touch on a daily basis, most of the time
they don't hurt us. And that is because we
have a great immune system which is diagrammed
here. We have an immune system that can fight
invaders such as viruses and other pathogens
and get rid of them before they cause a problem.
It's only when your immune system is down
that you have a problem with a virus infection.
So for example if you are immunosuppressed,
if you have a virus infection that is immunosuppressive
like HIV, other viruses will then cause problems.
03:17
If you take drugs to immunosuppress you, let's
say you've had an organ transplant, you get
drugs so that your body does not reject the
organ, your immune system is now compromised.
03:27
You can't fight off those smallest virus infection.
So we have our immune system to thank for
this wonderful balance between viruses being
in us and not doing anything and viruses causing
disease.
03:42
I would like to make the case that we already
have evidence that many viruses are good and
here is one example of that. There is a small
DNA virus diagrammed here called polyomavirus,
which seems to infect everyone, starting at
the young age throughout their lives. And as
far as we can tell, unless you're immunosuppressed,
these viruses don't hurt you at all. We think
they're probably beneficial, but we don't
have evidence for that yet. But the interesting
thing about these polyomaviruses is that because
they are transmitted within families, they
are markers of human migration. So for example,
we know that humans originated in Africa and
from Africa moved to Europe, to Asia and then
to the Americas, and that's shown by the dotted
line here. This is a migration pattern that
you can deduce by looking at the sequences
of human DNA. If you look at the polyomaviruses
that people carry, you get the same conclusion,
that humans originated in Africa, they moved
to Europe and we even have more detail when
we look at the polyomavirus tracing, we can
see that they went down into Australia and
Australasia and then over to North and South
America. So these are uses of a beneficial
virus and they are probably more that we remain
to discover as well.