http://theconversation.com/whats-the-difference-between-pandemic-epidemic-and-outbreak-133048
The coronavirus is on everyone’s minds. As an epidemiologist, I find it interesting to hear people using technical terms – like quarantine or super spreader or reproductive number – that my colleagues and I use in our work every day.
But I’m also hearing newscasters and neighbors alike mixing up three important words: outbreak, epidemic and pandemic.
Simply put, the difference between these three scenarios of disease spread is a matter of scale.
Outbreak
Small, but unusual.
By tracking diseases over time and geography, epidemiologists learn to predict how many cases of an illness should normally happen within a defined period of time, place and population. An outbreak is a noticeable, often small, increase over the expected number of cases.
Imagine an unusual spike in the number of children with diarrhea at a daycare. One or two sick kids might be normal in a typical week, but if 15 children in a daycare come down with diarrhea all at once, that is an outbreak.
When a new disease emerges, outbreaks are more noticeable since the anticipated number of illnesses caused by that disease was zero. An example is the cluster of pneumonia cases that sprung up unexpectedly among market-goers in Wuhan, China. Public health officials now know the spike in pneumonia cases there constituted an outbreak of a new type of coronavirus, now named SARS-CoV-2.
As soon as local health authorities detect an outbreak, they start an investigation to determine exactly who is affected and how many have the disease. They use that information to figure out how best to contain the outbreak and prevent additional illness.
Epidemic
Bigger and spreading.
An epidemic is an outbreak over a larger geographic area. When people in places outside of Wuhan began testing positive for infection with SARS-CoV-2 (which causes the disease known as COVID-19), epidemiologists knew the outbreak was spreading, a likely sign that containment efforts were insufficient or came too late. This was not unexpected, given that no treatment or vaccine is yet available. But widespread cases of COVID-19 across China meant that the Wuhan outbreak had grown to an epidemic.
COVID-19 was first noticed in Wuhan, China, in late 2019 but quickly spread across the globe. This map shows all countries with confirmed cases on March 5, 2020. CDC