Modes of Transmission and Disease Transmission Cycle

00:01 Let's talk now about the ways in which diseases are spread.

00:06 How do you contract them from other people? First, there's direct contact.

00:11 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.

00:26 Right, so HIV/AIDS classically, well known as spread via sexual contact or by a blood contact.

00:36 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.

00:49 Direct contact is literally what I talked about just now.

00:51 It's the fluids of one person coming into contact with the fluids of another person, often very intimately.

00:59 I was talking about droplets transmission The flu, influenza is transmitted via droplets.

01:06 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.

01:22 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.

01:32 So if you keep your distance, you're unlikely to receive one of those droplets incidents on your body.

01:39 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.

01:48 So any disease that is coughed forth or sneezed forth or breathed out can be transmitted via droplets.

01:55 COVID 19 primary mode of transmission is through droplets, but recent evidence shows that COVID 19 virus is also spread in an airborne manner.

02:04 That's why we have masks to slow and to diminish the distance by which a droplet can travel.

02:13 And also while we maintain our social or physical distances so that droplets cannot land on us.

02:20 Indirect contact or fomite contact or surface contact is an important way of transmission as well.

02:28 That's when an object is a vector that allows transmission to happen.

02:35 Imagine someone with the flu sneezes on their hand, then touches a door knob then walks away.

02:41 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.

02:50 The fomite is the door knob.

02:52 So inanimate objects can be the intermediary pathway of transmission.

02:59 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.

03:07 So hand washing becomes a very important method for mitigating the transmission of diseases that have a strong fomite component.

03:19 Airborne diseases are the scariest for a lot of people.

03:23 This is when the nuclei of dried droplets and aerosols can float into the air and linger there for a long period of time.

03:33 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.

03:48 But airborne nuclei can last for hours and sometimes days.

03:53 So airborne diseases are a bit scarier in that they are more likely to be transmitted very often.

04:00 Measles famously is airborne.

04:02 That's why it's so infectious.

04:06 vector transmission is when a living being is the mode by which a disease travels from an infected person to a susceptible host.

04:17 The classic example is a mosquito.

04:20 So the mosquito carries the Plasmodium parasite that it got from biting an infected person and then comes and bites you and delivers that parasite into your bloodstream.

04:33 So the parasite has no other way to get into you except via this vector.

04:38 So, if we can control the behaviors of these vectors, we can control the spread of the disease.

04:47 Let's talk now about the transmission cycle and I'll talk about the reservoir, the mode of transmission and susceptible host.

04:57 So the reservoir of the disease is the place where it lives usually.

05:02 There's a natural reservoir, that's where it retreats to when it's not busy infecting people.

05:08 or sometimes the reservoir we talk about is where it is right now, most commonly.

05:13 So if you are infected, you are a reservoir.

05:19 You're not the natural reservoir necessarily.

05:22 For example, the natural reservoir of Ebola is bats.

05:26 We think that's where it usually lives, but it's moved in people, So people become reservoirs as well.

05:33 How the reservoir gets out is the mode of transmission or how the pathogen gets out the reservoir is the mode of transmission.

05:39 We talked a bit about these modes of transmission already in direct contact, fomites, airborne, etc.

05:47 and they need a susceptible host to infect.

05:53 So there are many portals of entry into the body of a host.

05:56 The eyes are common.

05:58 The nose, the mouth, any place where there are mucous membranes.

06:01 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.

06:12 For transmission to happen, all three components must be present.

06:19 So now you can think about the various infection control methods that we can now intervene with.

06:26 We can remove the reservoir so that there are no sources of pathogens to begin with.

06:34 You could intervene between the reservoir and prevent the mode of transmission by taking the mosquitoes out of the picture for example.

06:42 You can intervene between the mode of transmission and the susceptible host.

06:46 For example, by implementing mask wearing.

06:49 Right? A really good mask will prevent many of the virus or the droplets from getting through, now you protected the host from transmission.

06:59 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.