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PROFESSOR: So now let's
talk about viruses,

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which is our main focus
in these lectures.

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So viruses are very
different from bacteria,

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because they are pathogens which
infect the cells of your body.

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And so they themselves are
not large cellular organisms,

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but in fact have a
different biology.

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So they consist
of virions, which

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are a capsid form of the virus,
which contains a strand of RNA,

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which is some genetic material
that, when this capsid is then

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integrated into the host cell,
the RNA can basically activate

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to infect that cell, and also
replicate itself and make

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some more virions, which
can spread out and infect

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additional cells.

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So the basic entity that
we're worried about here

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is really a small object, which
is often shaped like a sphere,

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or maybe it's an
ellipsoid, the virion.

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And it's much, much smaller.

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So the typical size is
really on the order --

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the radius, let's say -- is on
the order of 100 nanometers.

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So that is 0.1 micron or less,
so much smaller than even

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the smallest aerosol droplets
that we can easily observe.

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So they're extremely tiny.

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There's a couple examples
we can think about,

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which are famous examples.

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So one class of viruses
cause the disease measles.

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And measles has a virus
shape, as you can see here,

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which is not quite spherical but
usually a little bit elongated,

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like an ellipsoid,
and has a typical size

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of 100 to 300
nanometers in diameter.

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So measles is still a
very active disease,

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which we have been
controlling for a long time

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with vaccinations.

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But despite that, there
were still, in 2018,

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114,000 deaths
worldwide from measles.

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And it's estimated that in
the eight years before that --

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actually, in the 20 years
before that, excuse me --

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that the vaccine has saved
around 23 million lives that

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would be lost otherwise
if this virus were

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allowed to propagate.

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And measles -- it is
known to be airborne.

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So this is the classic example
of an airborne transmission.

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It's been studied
by, for example,

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Riley in the 1970s,
who demonstrated

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measles transmission in
schools and in other settings.

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And from our perspective,
in this lecture,

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it's not surprising.

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These viruses or
virions are so small,

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they can be contained in
the smallest droplets, which

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are easily present
in the air for hours,

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and so they are
not settling out.

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So what's of much more
interest to us today

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is the family of coronaviruses.

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So coronaviruses look
very similar to this one,

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but they have these
proteins that stick out,

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which we've all
seen, that look a bit

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like a crown, or a corona.

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And they still have
an RNA on the inside.

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And there's lots of different
human coronaviruses.

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So there are the standard
human coronaviruses,

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which cause the common cold.

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And there are four typical
human coronaviruses

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that cause common colds
that we all experience,

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and more serious pneumonias,
but generally not

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life-threatening illnesses.

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But there are variations of the
coronavirus mutations, which

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are constantly coming
into contact with humans

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and can cause much
more serious diseases.

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So in recent memory, we've had
the Severe Acute Respiratory

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Syndrome coronavirus.

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And there was a big outbreak,
reasonably big outbreak

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around 2003, that
started also in China.

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And it infected around 8,000
people, and about 800 died.

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So It was a fairly lethal
disease, about 10% mortality,

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but fortunately, it
didn't spread too widely.

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More recently, there was
the Middle East Respiratory

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Syndrome coronavirus,
which was in 2012.

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And this led, over the couple
of years after that as well,

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to around 2,500 cases
and around 850 deaths.

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So again, an outbreak that
was potentially very serious

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but remained controlled and
was primarily in Saudi Arabia.

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And then we come to SARS-CoV-2,
which, we all know the story.

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So this is the
novel coronavirus,

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which appeared in Wuhan,
China, in December 2019,

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and then led to the
present global pandemic,

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which so far has claimed
almost a million deaths.

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So as of today, which
is September 24,

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it's around 976,000
deaths worldwide.

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And the confirmed cases are
currently at about 3.8 million,

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which is a reasonable fraction
of the world's population,

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around 7 billion.

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So what we can
understand from all this

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is that these are
very small objects.

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They cannot swim.

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And so they are just sort
of floating in droplets.

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And so, of course, they
can be in small droplets.

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And as we'll discussed
next, you can

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imagine that they're
actually more

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infectious in those
small droplets,

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because they can more easily get
out and reach host cells when

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they're transmitted.

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Moreover, small
droplets are much easier

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to get deep into your
respiratory system,

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into the smallest
cavities of your lungs,

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where there's a high surface
area for that interaction

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to take place.