Epidemics: Types

00:01 So epidemics can occur from almost any type of disease.

00:05 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.

00:14 Things you can get from other people.

00:17 And we can classify epidemics according to the way in which they spread through a population.

00:23 In general, there are three types of epidemics.

00:26 Common-source epidemic, and a common source epidemic has three subtypes: point-source, continuous or intermittent.

00:34 The second type is propagated.

00:36 And lastly, you can have a combination of common source and propagated types.

00:42 Let's go through them.

00:45 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.

01:02 If someone brings tainted chicken and you all eat the chicken and you all get sick from it.

01:06 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.

01:26 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.

01:50 So you can identify it that way.

01:53 The continuous, common-source epidemic, on the other hand, is when people are exposed over a longer period of time.

01:59 So the cases don't occur over the course of one incubation period.

02:03 An example here would be tainted water.

02:06 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.

02:24 The intermittent common-source epidemic is when people are exposed over a period of time, but not continuously.

02:32 They're exposed intermittently, hence the word.

02:34 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.

02:43 And that depends on how long the exposure lasts.

02:46 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.

03:01 But the factory isn't dumping pollutant every day, just every few weeks.

03:05 So intermittently, the villagers will be exposed to it.

03:09 It's the same source, but they're not continuously being exposed, just intermittently.

03:15 A propagated epidemic on the other hand, is one that evolved spread from person to person.

03:22 And this is how most of us tend to think about infectious diseases, the things that we get from other people.

03:27 So influenza, that's a propagated epidemic.

03:30 You get it from your neighbor, you get it from your friends, you get it from your, someone you share your home with.

03:36 COVID-19 of course, is a propagated epidemic.

03:39 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.

03:50 It's possible to have mixed epidemics.

03:52 So some epidemics have features of both common-source and propagated.

03:56 And an example of that would be someone who is exposed to shigellosis.

04:01 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.

04:12 Well, they got it from a common source.

04:15 That's the river is the source that many people can drink from.

04:18 But then these people go back home and they're making food for their friends and they contaminate them that way.

04:24 So suddenly they are propagating this infection that they got from another source that in itself is also a source of infection for others.

04:34 So unfortunately, many epidemics have a mixture of features which makes it difficult to sometimes put them into easy categories.