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PROFESSOR: So now
that we've talked

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about the different dynamics
of droplets in the air,

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we can think about how
different types of pathogen

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can leverage those
droplets to transmit

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from one person to another
through respiration

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through the air.

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So let's begin with bacteria.

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So there are many different
kinds of bacteria.

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The typical size of
a single bacterium

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is on the order of
several microns.

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So let's just say,
1 to 10 microns.

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On the other hand,
bacteria can also

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exist in colonies or
larger structures.

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And that does determine,
to some extent,

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what kinds of droplets can
transmit those bacteria.

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So let's begin with an
example of a large-drop

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bacteria, which
is typically going

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to be found in large drops.

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And that would be
the bacteria that

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causes strep throat,
the streptococcus.

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So the streptococcus
bacteria, which is shown here,

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has a typical size that's
around two microns.

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So it's on the smaller end.

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And it forms chains and
even larger colonies they

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may have different structures.

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And the total size
of the colonies

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can vary to be as big
as even 500 microns.

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So it may not be always
that big, but it might be,

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let's say, 50 to 500
micron sort of colonies.

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And they can often be these
sort of stringy type structures.

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As a result, those require
fairly large droplets.

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Now just simply knowing
that we have large droplet,

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tells us a lot about how this
bacteria can actually transmit

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and what measures must
be taken against it.

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Because as soon as we have
large drops, then what it means

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is that we have fast settling.

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So we know drops that
are in this size range

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are going to fall to the
ground within a few seconds.

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And even if you cough them
with a lot of big velocity --

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which is something we'll
talk about later in this

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and these lectures --

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then still, in
only a few seconds

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those droplets can sediment
out and they really

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don't go very far.

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OK?

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And so they're fast-settling.

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And so what that
means is that we have

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either fomite transmission,

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where those droplets
settle on the ground,

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or on some other surface --

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somebody touches that
surface, touches their eyes --

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and so that's one
method of transmission.

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We could also have direct
airborne transmission.

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Where let's say, I
cough or breathe on you.

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And the droplets end up in
your face and maybe directly

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on your eyes,

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or maybe you breathe them in --

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so there could be also
direct airborne transmission

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for some of the
smaller sized droplets

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or for large droplets which are
ejected from let's, say a cough

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or something.

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And so what that means,
that the way that we protect

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against bacterial
transmission of disease

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is, for example,
for the fomites,

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we will disinfect surfaces.

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We can wash hands, of course.

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And we will avoid
touching eyes or nose.

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So those are pretty
basic measures.

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At the same time,
if we're worried

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about the direct
transmission of the droplets

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from especially coughing or
sneezing larger droplets,

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then what else can we do?

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Well, we can have
plastic shields.

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Either worn over
the face or maybe

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a barrier between yourself and
some other person that you're

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interacting with to avoid that
sort of projectile transmission

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of large droplets.

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And also, we will get
the six-foot rule.

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Which is just an example of
a social distancing measure,

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which is recommended
to avoid this sort

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of direct airborne transmission.

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Again, coming back to the
idea that droplets of the size

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typically settle
in a few seconds.

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And if you look at the typical
velocities of ejection,

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especially from
coughs and sneezes,

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then they will settle in about
six feet or so or about two

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meters.

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Although that's not a hard rule.

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The six-foot rule happens to be
for the United States Centers

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for Disease Control, the CDC.

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But there's also [the]
one-meter rule, which

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is basically a three-foot
rule, for the World Health

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Organization.

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So this is not
extremely well defined,

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what should be the distance.

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But clearly, if you are able
to stay away from people,

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then even if they're
coughing or breathing

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these large droplets with only
a few seconds of settling time,

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those droplets
will hit the floor.

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And it will not be able
to infect you directly.

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And so it's
important to maintain

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that kind of distancing.

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OK.

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Now there are also other
kinds of bacteria that don't

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form these larger colonies.

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And they remain small even
when they're transmitting.

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And a classic
example of that which

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can be transmitted in small
drops is tuberculosis.

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In fact, the original
study in the 1930s --

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also involving wells -- was for --

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that led to this --

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essentially the distancing
rules and the six-foot rule

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in particular -- had to do
with coughing and sneezing

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and the distance over which
droplets could be transmitted

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containing tuberculosis.

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Now tuberculosis
on the other hand,

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as you can see in
the image, is a sort

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of rod like bacteria
that's quite small.

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So the length is several
microns, typically two to four.

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But the radius is half a micron
or even down to 0.2 microns.

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So actually, we're really
talking about hundreds

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of nanometers in length.

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So these are kind of
little rods like this.

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And they can be contained
in a pretty small droplet.

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These big colonies of course,
require something much larger.

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So these droplets
could be small.

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And so there is a
possibility here

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of larger aerosol transmission.

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By larger, I mean in the
range of 5 to 10 microns.

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So of course
tuberculosis also can

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be contained in much
larger droplets, which,

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you know, fall to the ground
as we've just been describing,

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but tuberculosis
is a bit different

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in that these individual
bacteria could

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be transmitted airborne in
larger aerosol droplets.

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And in fact, that
is what is found.

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If you have this
size of droplet,

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then it turns out, for
example, if my radius is

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let's say greater
than 4 microns,

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then the settling
velocity you can

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find given the density of
aqueous fluids, is around 2 --

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bigger than 2
millimeters a second.

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And then the time to drop
from a typical person's height

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to the ground is
around 15 minutes.

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So these are not
the kind of aerosols

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that might stick
around for hours

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like the sub-micron
aerosols that are also

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produced by breathing
and which are too

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small to contain tuberculosis.

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But still, these times
suggests that tuberculosis

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can linger in the air.

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And in fact, long distance
airborne transmission

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is not only possible
as we've just

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argued based on physical
grounds, but it's

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been directly verified.

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That's been done both
in humans studies

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and also in animal studies.

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Where they can have two
different compartments with

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a sick animal, such as a ferret
or some other animal model,

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and the disease is
spread to another animal

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who's had no direct contact
but is sharing the same air.

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So the airborne
transmission is definitely

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verified in this case.

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And now you might
ask, well, what

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are the preventive measures?

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So instead of all
of these measures,

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like plastic shields
and six-foot rule,

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if you're actually
airborne transmission --

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if there is a
shield, then the air

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is going to go right
around the shield.

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And it's going to go
everywhere else in the room,

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because the air is flowing.

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In the same way that when
you light a candle and you

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see where the smoke
is going, it quickly

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spreads around the whole room.

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It doesn't care if
there's a shield there.

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As long as there is a
way around the shield,

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that's going to find that.

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And so instead,
when you're trying

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to protect against
airborne transmission,

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your protective measures
are things like ventilation,

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so bring in fresh
air from outside

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or opening your windows;

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air filtration, where
you're passing the air

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through a filter which
is going to filter out

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the particles by size or by
charge or some other mechanism

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like that;

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and also it becomes
more and more

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important to wear face masks

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because then at least you
can block these droplets

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at the source and
also at the target,

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as we shall discuss
in much more detail.

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So what we see here
is just knowing simply

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the size of the pathogen
and understanding

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the physics of
droplets in the air,

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helps us to understand
the modes of transmission,

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which are written here in blue.

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and also, the appropriate
protective measures,

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which are written here in pink,

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to protect against
these different types

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of transmission.