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