epidemiological curves 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
going to talk about how to interpret
them how to make sense of them and of
course I'm going to 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 the University of Notre Dame
run a Masters of 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 epic curve or an
epidemic curve an epidemic curve is a
visual representation of the onset of
illness of cases in an outbreak over
time we're going to talk about three
kinds of outbreaks the first is a
point-source outbreak and that's when a
population or 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 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 going to 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 a period 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 let'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 curves 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 first two
types of outbreaks are a function of
environmental exposure the point source
and the common source now let's talk
about what would happen if we have
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 a person can actually be
spreading the disease before they become
ill and 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 at
the virus or the microbe is too lazy to
spread an incubation period starts
within and after the incubation period
you have to stay in your house or in
your bed 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 going to call him John and at
some point in time John becomes infected
with a flu virus and at this point in
time of course John doesn't feel unwell
so he goes off to work and he's
surrounded by his co-workers 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 Dez becoming
fix's so that's the end of a latent
period and at the end of the latent
period he is 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 flu virus on to
his coworkers now let's take a look at
what this exact same scenario looks like
on an AP curve so John becomes sick
after a period of time he becomes
symptomatic at the point at which he
becomes symptomatic or ill we see him
pop up on the AP curve 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 at the
population that were exposed in this
case the point in time at which they
were exposed preceded the first block on
the AP curve 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 curve and more and more of
John's co-workers 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 infectious period will 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 become
infectious and they become infectious
before they become symptomatic they each
have an infective 'ti 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
respective spikes on the EP curve 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
there's 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 dead 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
there may be public
of control measures put in place things
like education treatment and maybe even
quarantine to float the onward spread
and the onward transmission of the
disease and if the disease does not
peter out completely it may become what
we call endemic and that means that
there's low-level ongoing transmission
within a community and that's more or
less how to interpret AP curves in a
nutshell thank you for watching I hope
you found that useful remember to
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bit more about the University of Notre
Dame's Master of Science in global
health program for those of us that are
already working in the global health
space or in the public health space you
and I both know that from time to time
people ask us about what they should
study where they should study and in
that sense I want you to think about
learning more about the University of
Notre Dame's Master of Science and
global health program it produces
graduates that have an excellent
understanding of the science that
underpins the practice of global health
thanks for watching see you
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