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Today we're going to talk about communicable diseases.
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More specifically, we're going to explore ways in which
diseases are spread.
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We call these modes of transmission.
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The first type of disease transmission that we'll discuss is
direct transmission.
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Direct transmission occurs by immediate transfer of
infectious agents from a reservoir,
in this instance, an infected person, to a new susceptible
host.
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And as you can see here, we have a healthy, uninfected
person as the host.
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Direct transmission can happen in two different ways.
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It can be through either direct contact or droplet spread.
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Direct contact includes direct contact with oral secretions,
blood, or other potentially infectious fluids
such as drainage from a skin lesion, semen, or vaginal
secretions.
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These can happen as a result from touching, biting, kissing,
or sexual intercourse.
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Then, there's droplet spread which is another form of direct
transmission.
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For droplet spread to occur, there needs to be direct
contact with infected droplets
that contain the infectious agent.
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This could occur as a result of an infected person coughing
or sneezing secretions
directly into the face of an uninfected person.
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This can transmit respiratory infection such as pertussis,
or what we call, the whooping cough, by way of droplet
spread.
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In order for this to occur though, people need to be in
close proximity to each other.
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This means that in order for droplet spread to happen,
people need to be less than 2 meters from one another.
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This could happen in instances where people are sharing a
space
such as a car or together in a small room.
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Now, remember, in order for infection to be spread through a
droplet,
an individual would need to be in close contact with the
infected individual.
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In the next couple of slides, I'm gonna explain how this is
different than airborne transmission.
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But before we move on, I want you to remember, that both
direct contact
and droplet spread are forms of direct transmission.
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Next, we have indirect transmission,
and once again, there are several types of indirect
transmission.
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One of these is airborne transmission.
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Airborne transmission occurs when an infectious agents are
carried by dust, suspended in the air.
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Because of the small size and the weight of the droplet
nuclei,
they can remain suspended in air for long periods of time
before they're inhaled into the respiratory system of a
host.
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With airborne transmission, direct contact or even close
proximity is not needed to spread disease.
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With airborne transmission, droplet nuclei remain airborne
for long periods of time.
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They may disseminate widely in an environment
and they can be acquired by a host even if they don't have
direct contact with the infected person.
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So let's take a look at a picture that'll really help
differentiate between the two.
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Here we see two different diseases, both carried and spread
in the respiratory tract.
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But we can see the difference between the Ebola virus
disease
which is only spread through direct contact through droplet
spread.
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See, here, the droplets are large, they're heavy.
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They don't stay suspended in the air for long periods of
time.
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In order to be infected, you would need to be in close
contact with an infected person.
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On the other hand, we have the measles. These infectious
agents are lightweight, they're small,
they stay suspended in the air for long periods of time, and
can travel over long distances.
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In this case, you could come in contact with the agent by
simply being in the same say
building or an apartment or restaurant as an infected
person.
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Hopefully, this picture helps differentiate between droplet
spread,
which is again, is direct transmission, and airborne spread,
which is a form of indirect transmission.
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And then we have another type of indirect transmission,
vehicle-borne transmission.
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This type of indirect transmission occurs when an infectious
agent
is transported within a contaminated inanimate material.
These are called fomites.
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Examples include toys, sharp objects, tissues, even food and
water.
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As I mentioned, food and water are also fomites and can
spread illnesses through indirect transmission.
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This type of illness occurs when food or water is
contaminated.
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This can occur at the source.
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For example, animal waste can be introduced directly into a
food or water source, contaminating it.
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This can also occur due to improper handling of food or
improper food storage.
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Contamination of food that leads to human illness occurs in
two different ways.
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Either through infection or through intoxication.
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Infection occurs when a food has been contaminated directly
by the pathogen and that pathogen is ingested.
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An example of this would be ingesting food that's been
contaminated by e-coli or salmonella.
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The human is infected by direct consumption of that
pathogen.
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On the other hand, intoxication occurs when the food that's
ingested has been contaminated by a toxin,
and that toxin is a by-product of the pathogen.
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For example, I'm sure you've all heard of botulism.
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Botulism intoxication occurs as a result of a by-product of
the growth of the bacteria,
not as a result of the bacteria itself.
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This is the image depicting the infamous cholera outbreak of
the 1850s in London.
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There was a cholera outbreak which killed more than 600
people.
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At that time, germ theory was not widely accepted.
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We didn't know why people were suddenly getting sick and
dying.
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John Snow was credited by determining the source,
contaminated water.
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He mapped out the cases of the disease to pinpoint the
source,
he removed the handle of the Broad Street pump, and the
epidemic ended.
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This is one of the first documented cases of indirect
transmission of water-borne illness.
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Another form of indirect transmission is vector
transmission.
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A vector is a living organism that transmits an infectious
agent
from one infected animal to a human or another animal.
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Vectors are frequently anthropods, such as mosquitos, ticks,
fleas, lice.
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Vectors can transmit infectious diseases either actively or
passively.
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Biological vectors such as mosquitos and ticks, may carry
pathogens
that can multiple within their bodies and be delivered to
new hosts, usually by biting.
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Mechanical vectors on the other hand such as flies,
pick up infectious agents on the outside of their bodies and
transmit them through physical contact.
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Vector-borne illnesses are spread when an infected
individual transmits the disease to a vector.
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The vector then transmits that disease to another
individual.
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However, without the vector, the individuals are not able to
transmit disease to each other.
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Of all the infectious diseases, vector-borne illnesses are
the most complex to prevent and control.
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Control strategies directed towards vector-borne diseases,
typically involve community education and environmental
measures
to hinder the vector from reaching its host.
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For example, strategies to protect individuals against
malaria,
which is transmitted through vector spread, through
mosquitos, include vaccinations.
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Environmental measures such as the use of mosquito nets or
eliminating standing water.
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In summary, modes of transmission are ways that communicable
diseases are spread.
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We have direct transmission that includes direct contact and
droplet spread.
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Next, we have indirect transmission.
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That includes airborne-spread, vehicle-borne transmission,
and vector transmission.