So epidemics can occur from
almost any type of disease.
We talk about the obesity epidemic or the
cancer epidemic or the hypertension epidemic,
but we usually use the word to
describe outbreaks of infectious diseases.
Things you can get from other people.
And we can classify epidemics according to the
way in which they spread through a population.
In general, there are three types of epidemics.
Common-source epidemic, and a
common source epidemic has three subtypes:
point-source, continuous or intermittent.
The second type is propagated.
And lastly, you can have a combination
of common source and propagated types.
Let's go through them.
A common-source epidemic is an outbreak
in which a group of people are all exposed
to the same infectious agent or toxin
from the same source, like tainted food,
so a picnic, which you go with
your co-workers, for example.
If someone brings tainted chicken and you
all eat the chicken and you all get sick from it.
Well, that would be a common-source
outbreak or common-source epidemic
because the common source
is the chicken from the picnic
In fact, the example I just gave
you is a point-source common-source
because you all got sick from
the same thing at the same time
and this is very common in outbreak epidemiology.
It's something that public health
epidemiologists are always investigating,
small outbreaks of diarrhea or salmonella
or some other common infectious disease
happening in a city any given time
in any major city that's happening
With point-source, common-source
epidemics, all the cases tend to fall within
one incubation period of the disease.
So you can identify it that way.
The continuous, common-source
epidemic, on the other hand,
is when people are exposed
over a longer period of time.
So the cases don't occur over
the course of one incubation period.
An example here would be tainted water.
If the water source of a neighborhood
or a community is tainted with
E. coli or some other kind
of disease causing diarrhea,
then over the course of many weeks or
months, everyone in that area will get sick
and they're getting it from
the same common source
but over a longer period of time.
The intermittent common-source
epidemic is when people are exposed
over a period of time, but not continuously.
They're exposed intermittently, hence the word.
So onset will still be abrupt, but the cases
will be spread over a greater period of time
and over more than one incubation period.
And that depends on how long
the exposure lasts.
An example of this would be if a
village is downstream from a factory
that is dumping pollutants into the water
and that villages using that river water
for drinking and other purposes,
they might get sick from it.
But the factory isn't dumping
pollutant every day, just every few weeks.
So intermittently, the
villagers will be exposed to it.
It's the same source, but they're not
continuously being exposed, just intermittently.
A propagated epidemic on the other hand, is
one that evolved spread from person to person.
And this is how most of us tend
to think about infectious diseases,
the things that we get from other people.
So influenza, that's a propagated epidemic.
You get it from your neighbor, you get
it from your friends, you get it from your,
someone you share your home with.
COVID-19 of course, is a propagated epidemic.
So in propagated epidemics, cases occur
over more than one incubation period
because they're always propagating
and starting over in a new susceptible host.
It's possible to have mixed epidemics.
So some epidemics have features of
both common-source and propagated.
And an example of that would be
someone who is exposed to shigellosis.
Let's say they drank from a contaminated
water source when camping one day and
and drank water they should
not have drunk from, a river
and they were exposed to
shigellosis and got infected.
Well, they got it from a common source.
That's the river is the source
that many people can drink from.
But then these people go back home
and they're making food for their friends and
they contaminate them that way.
So suddenly they are propagating this
infection that they got from another source
that in itself is also a
source of infection for others.
So unfortunately, many
epidemics have a mixture of features
which makes it difficult to sometimes
put them into easy categories.