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?
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.
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 .