What are the differences between an epidemic and an outbreak? Both words mean unexpected or unusual occurrence of the disease. Can someone explain the difference between them?

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    It isn't difficult to do some research and present what you've found when asking here; doing so would be helpful in getting a response. Please see Is it OK to ask questions that don't show any research?. Thanks. Commented Jan 27, 2016 at 18:50
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    Also, "epidemic" does not mean an "unexpected or unusual occurrence of the disease," so I would recommend beginning your research with proper definitions from a dictionary.
    – Carey Gregory
    Commented Jan 27, 2016 at 21:27
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    I didn't mean for a research. Just would like to know the different usage of these two worlds. Thanks!
    – user2756
    Commented Jan 28, 2016 at 1:13
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    @anongoodnurse I don't actually think this question indicates a lack of research, but rather a brevity of language. Which isn't necessarily a bad thing. That this question, before I upvoted it, was tied for the worst question score in Top Questions makes me thing worrying things about the site as a whole.
    – Fomite
    Commented Jan 29, 2016 at 19:43
  • @Fomite - As with the opinion of the question's quality, interpreting the absolute lack of evidence of research as brevity is subjective. This state of site as a whole is, in my opinion at least, something to be concerned about, and not only because of this particular question. Recent meta and waiting room discussions have been addressing this issue. The front page is awful. That concerns me. Commented Jan 29, 2016 at 23:13

2 Answers 2


As a practicing epidemiologist, I'm actually a little puzzled as to the down-votes for this. The difference between an outbreak and an epidemic is pretty subtle, and neither is all that terribly well defined.

The answer: There's not much of a difference.

The CDC agrees with me:

Occasionally, the amount of disease in a community rises above the expected level. Epidemic refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area. Outbreak carries the same definition of epidemic, but is often used for a more limited geographic area.

In practice, I'd argue that an outbreak is, as they mentioned, a somewhat smaller-scale phenomena. I'd talk about an outbreak of norovirus at a university, while I'd say a state, large city or country was having an epidemic.

I'd also say that, for very low levels of disease, it's possible to have an outbreak without an epidemic. For example, in a paper I wrote, I characterized the cases of disease in a population as "Low-level endemic spread, punctuated by periodic, short-lived outbreaks". These outbreaks were on the scale of less than a dozen people, so I'd really have a hard time calling it an epidemic, but again, this is referencing the idea that "outbreak" refers to a somewhat smaller geographical scale.

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    The downvotes are for a lack of research, that's all. We're trying to get users to show a tad of effort. On EL&U, people need to explain/show that they've looked a word up in a dictionary before asking about it. On Biology, lack of any research is closed as homework. Skeptics requires sources (for questions and answers.) There's little virtue in building an answer from the ground up when the OP can meet us a little bit of the way. And of course, to an epidemiologist this will be more nuanced than to the average person. Commented Jan 29, 2016 at 23:04
  • I agree with this answer, by the way. Much of medicine and the terms used to describe various phenomena are more ambiguous than most people realize. Commented Jan 29, 2016 at 23:10
  • We should probably be having this conversation in meta or in chat, but as it seems to be taking place here, I thought I'd weigh in. I agree it's generically a fine question. When I google the 1st sentence, the CDC page you quote is the second hit. I would have come in for an answer if that had been linked to, seeking further information. I'm uncertain if a basic level of research should be done, but I think it's not a bad idea (∴ my meta posts). In the meantime it doesn't seem to be policy ∴my lack of vote 1 way or the other. As you seem to come from the other side, I would value your 2 cents.
    – Atl LED
    Commented Jan 30, 2016 at 2:47
  • @AtlLED In my own wandering search, I encountered some contradictory stuff, and didn't get a clear definition to support my answer for hits. Though in fairness, I was using Bing.
    – Fomite
    Commented Jan 30, 2016 at 3:06

An outbreak is the sudden occurrence of a disease in a community, which has never experienced the disease before or when cases of that disease occur in numbers greater than expected in a defined area. The current Ebola scenario in West Africa started as an outbreak, which initially affected three countries. So what exactly is an epidemic? It is an occurrence of a group of illnesses of similar nature and derived from a common source, in excess of what would be normally expected in a community or region. A classic example of an epidemic would be Severe Acute Respiratory Syndrome (SARS). The epidemic killed about 774 people out of 8,098 that were infected. It started as an outbreak in Asia and then spread to two dozen countries and took the form of an epidemic. The same is true for Ebola, which is now being termed an epidemic .

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