What are they?

How do you use them?

How do you make them?

This is the second in a two part video series.

In the first video, we talked about how to make an epidemiological curve using Microsoft Excel.

If you haven't watched that, go back and watch it right now.

And in this video, we're gonna talk about how to interpret them, how to make sense off them.

And, of course, I'm gonna revise briefly what epidemiological curves are.

Hello, And welcome back to this global health YouTube channel.

My name is Greg Martin.

I just want to quickly send a big thank you to the University of Notre Dame for sponsoring and supporting this video.

I couldn't do it without you.

Thanks very much for those of you that are watching if you're interested in studying global health and getting a good sense of the science that underpins global health practice, University of Notre Dame run and martyrs off science in Global Health program.

That is absolutely outstanding.

I'll talk a little bit more about it at the end of the video.

So, firstly, what is an epi curve on epidemic curve?

An epidemic curve is a visual representation off the onset of illness of cases in an outbreak.

Over time, we're gonna talk about three kinds of outbreaks.

The first is a point source outbreak, and that's when a population of a group of people are exposed to a pathogen at a single source at a point in time or over a very brief period of time.

That's kind of like if if a group of people attend a wedding and there's some dodgy food and they all get sick a few days later and the second is a common source outbreak, and this is when people are exposed to some sort of environmental hazard that takes place over a period of time, like if there's pathogens in a water source.

And finally we're gonna talk about propagated outbreaks, and that happens when there's an infectious agent that spreads from person to person.

So if you look at the example that's on the screen right now, that's a point source outbreak.

Everybody was exposed at one point in time.

Now you'll notice in this example that despite the fact that everybody got exposed at the same point in time, they all attended the same wedding they all ate the same food.

Not everybody got sick at the same time.

The reason for this is that there's a lot of variation between people in terms of, firstly, the extent of their exposure.

So some people might have had more of the cake than others and their individual susceptibility.

Now the period of time between people being exposed to the pathogen and actually becoming ill is called the incubation period for each infectious agent.

We know that there's a period of time during which nobody gets ill, even though they've been exposed.

We call that the minimum incubation period, and of course, there's a maximum incubation period, and that's appeared of time, after which nobody gets ill.

So the epidemic curve really is a distribution of incubation periods.

Now let's imagine that this wasn't a point source outbreak.

It's imagine that this was a common source heartbreak, and basically that means that the population was exposed to the pathogen over a period of time like, for example, if the water source had pathogens in it.

So the important thing here in terms of epidemic curbs is that the population is exposed for an extended period of time and remember that in a point source outbreak, the people continue to get sick until the end of the maximum incubation period.

So in a common source outbreak, because the period of time during which people are exposed is extended, the point at which people stop getting ill will be the maximum incubation period from the end of the exposure, right, So those 1st 2 types of outbreaks are a function off environmental exposure.

The point source in the comet soars.

Now let's talk about what would happen if we have a person to person spread.

So what we call a propagated outbreak.

Our index case gets infected, and after a period of time, he becomes symptomatic.

We've already said that between the period of being infected and becoming symptomatic, we call that the incubation period, and there's a period of time during which that person is infectious.

In other words, they can pass it on to other people.

And, as you can guess, that's called the infectious period.

Importantly, the infectious period doesn't necessarily start after the person becomes symptomatic person can actually be spreading the disease before they become ill in the period of time between becoming infected and becoming infectious is what we call the latent period.

So to remember this, remember that the word latent starts with L.

L stands for lazy.

During that period of time, the virus or a microbe is too lazy to spread, and incubation period starts within.

And after the incubation period, you have to stay in your house or in your bid.

Now let's look at that exact same scenario, but in a slightly different way.

Let's imagine that we've got an index case.

We're gonna call him John, and at some point in time, John becomes infected with the flu virus.

And at this point in time, of course, John doesn't feel and, well, so he goes off to work and he's surrounded by his coworkers.

At this point in time, remember that John isn't ill, so he's still within the incubation period, and at this point in time, he's not infectious, so it's still within the latent period.

After a period of time, John does become infectious, so that's the end of the latent period, and at the end of the latent period, he's now spreading the disease.

So, of course, his co workers start getting sick and After that, he becomes symptomatic.

That's the end of the incubation period.

He goes home, he goes to bed.

But at that point, it's too late.

He's already spread the virus on to his co workers.

Now let's take a look at what this exact same scenario looks like on an epic of So John becomes sick.

After a period of time, he becomes symptomatic at the point at which he becomes symptomatic a will.

We see him pop up on the epic of, but we know that John was infectious before he became ill.

So the point in time when his co workers or the people at work, the people that the population that were exposed in this case, the point in time at which they were exposed preceded the first block on the epic Earth.

And after the minimum incubation period, following the point in time in which the co workers were exposed, the co workers start becoming symptomatic, and they start popping up on the epic of and more and more of John's coworkers.

And maybe his family as well will become infected during the entire length of his infectious period.

That's the entire blue Line and after John's and fixtures period.

We'll continue to see people getting sick, and they'll get sick at a point in time.

That corresponds with their respective incubation periods, and those incubation periods we know will continue until the maximum incubation period.

But of course, that's not the end of the story, because those co workers themselves becoming fixtures and they become infectious before they become symptomatic.

They each have an infectivity period, and so the virus begins to propagate one generation to the next.

And so we seeing this entire cycle repeat itself.

But as it repeats itself, we have more people that are infectious.

Initially, it was just one person.

Now there's a group of people, and so the respect of spikes on the epic earth get higher and higher, and these peaks eventually start to coalesce or merge into a wave, and then eventually the epidemic will begin to die off and burn itself out.

So why is it that these epidemics tend to burn themselves out?

Great question.

I'm glad you asked.

He has the answer.

In most outbreaks, the pathogens tend to run out of susceptible people, people that can be infected and is a few reasons for this.

Firstly, there'll be people in the population who gets sick and recover and are then immune and are no longer susceptible to be getting sick again.

Secondly, if it's a deadly disease, there may be people that die and of course, did.

People can't get sick.

Thirdly, as the outbreak unfolds, more and more of the population are in fact sick or infected.

And those people are, of course, not candidates to be infected.

And finally and importantly, they may be public health control measures put in place things like education, treatment and maybe even quarantine too fraught.

The n word spread and onward transmission off the disease.

And if the disease does not peter out completely, it may become what we call endemic.

And that means that this low level, ongoing transmission within a community and that's more or less how to interpret it curves in a natural Thank you for watching.

I hope you found that useful.

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