Let's talk now about the ways
in which diseases are spread.
How do you contract them from other people?
First, there's direct contact.
That's what a lot of people
first think about you know,
touching each other, that
includes sexual contact
contact with body fluids - saliva, blood,
urine, mucus secretion, feces, semen, etcetera.
Right, so HIV/AIDS classically, well known as
spread via sexual contact or by a blood contact.
And the flu is spread by droplets,
touching your mucous membranes,
that's not direct contact though,
that's indirect contact,
we'll get that in the second.
Direct contact is literally
what I talked about just now.
It's the fluids of one person coming into
contact with the fluids of another person,
often very intimately.
I was talking about droplets transmission
The flu, influenza is transmitted via droplets.
This is when small infectious
units, bits of virus or bacterium
depending on what disease we're talking about
are coughed up or sneezed
forth from deep within your lungs
and they tend to travel a certain
distance before falling to the ground.
With influenza, many studies
have been performed
that have determined that
these droplets tend to fall within
1 or 2 m of an infected person.
So if you keep your distance,
to receive one of those droplets
incidents on your body.
You don't want them falling on
your face where it could enter your
eyes or your mouth and your
nose, your mucus membranes
and gain entrance into your body that way.
So any disease that is coughed
forth or sneezed forth or breathed out
can be transmitted via droplets.
COVID-19 is thought to be transmitted this way.
That's why we have masks to
slow and to diminish the distance
by which a droplet can travel.
And also while we maintain
our social or physical distances
so that droplets cannot land on us.
Indirect contact or fomite
contact or surface contact
is an important way of transmission as well.
That's when an object is a vector
that allows transmission to happen.
Imagine someone with the
flu sneezes on their hand,
then touches a door knob then walks away.
When someone else touches that
same door knob then touches their face
and all the virus enters through
their mucous membranes in their face
and now they're infected.
The fomite is the door knob.
So inanimate objects can be the
intermediary pathway of transmission.
We don't think this is a major
way that COVID-19 is transmitted,
but we do think this is a common way
for the common cold to be transmitted.
So hand washing becomes
a very important method for
mitigating the transmission of diseases
that have a strong fomite component.
Airborne diseases are the
scariest for a lot of people.
This is when the nuclei of
dried droplets and aerosols
can float into the air and linger
there for a long period of time.
So droplets, we breathe forward droplets,
we cough them out, we sneeze them out,
they again fall to the ground after a certain
distance and do not linger for more than
a few minutes in the air
before falling to the ground.
But airborne nuclei can last
for hours and sometimes days.
So airborne diseases are a bit scarier in
that they are more likely to be transmitted
Measles famously is airborne.
That's why it's so infectious.
vector transmission is when a
living being is the mode by which
a disease travels from an infected
person to a susceptible host.
The classic example is a mosquito.
So the mosquito carries the
that it got from biting an infected person
and then comes and bites you and
delivers that parasite into your bloodstream.
So the parasite has no other
way to get into you except via this vector.
So, if we can control the
behaviors of these vectors,
we can control the spread of the disease.
Let's talk now about the transmission
cycle and I'll talk about the reservoir,
the mode of transmission and susceptible host.
So the reservoir of the disease
is the place where it lives usually.
There's a natural reservoir, that's where it
retreats to when it's not busy infecting people.
or sometimes the reservoir we talk about
is where it is right now, most commonly.
So if you are infected, you are a reservoir.
You're not the natural reservoir necessarily.
For example, the natural
reservoir of Ebola is bats.
We think that's where it usually lives,
but it's moved in people, So
people become reservoirs as well.
How the reservoir gets out
is the mode of transmission
or how the pathogen gets out the
reservoir is the mode of transmission.
We talked a bit about these modes of transmission
already in direct contact, fomites, airborne, etc.
and they need a susceptible host to infect.
So there are many portals of
entry into the body of a host.
The eyes are common.
The nose, the mouth, any place
where there are mucous membranes.
Getting through the skin is difficult,
but if there is trauma, if there's a cut
or if a mosquito bites through or syringe
places it through, then that's possible too.
For transmission to happen, all
three components must be present.
So now you can think about the
various infection control methods
that we can now intervene with.
We can remove the reservoir so that there
are no sources of pathogens to begin with.
You could intervene between the reservoir
and prevent the mode of transmission by
taking the mosquitoes out
of the picture for example.
You can intervene between the mode
of transmission and the susceptible host.
For example, by implementing mask wearing.
Right? A really good mask
will prevent many of the virus
or the droplets from getting through, now
you protected the host from transmission.
So it's useful to think about the nature of
transmission cycles as a way to think about
creative ways of implementing
infection control mitigation strategies.