Epidemic and pandemic are used to describe widespread outbreaks of a disease, but there are subtle differences between the two words.
An epidemic (from Greek epi- upon + demos people) is a classification of a disease that appears as new cases in a given human population, during a given period, at a rate that substantially exceeds what is "expected," based on recent experience (the number of new cases in the population during a specified period of time is called the "incidence rate").
A pandemic (from Greek πᾶν (pan, “all”) + + δῆμος (dēmos, “the people”) is an epidemic that spreads across a large region (for example a continent), or even worldwide.
Differences Between an Epidemic and a Pandemic
Simply put, when an epidemic gets out of hand, it is called a pandemic. This has 2 nuances:
- Geographical spread
- An epidemic that is not localized to a city or a small region but spans a larger geographical area can be called a pandemic.
- Incidence rate
- An epidemic may be localized to a small region but the number of people affected may be very, very large compared to what is "expected". In this case, it can be called a pandemic even if its geographical spread is not very large. For example, let us say that a disease has an "expected" rate of infection of 15%. When 40% of the population of a state is infected, we have an epidemic on our hands. When 75% of the population is infected, it has reached pandemic proportions.
Video Explaining the Differences
In this video, Dr. Kenneth Alexander, chief of pediatric infectious diseases at the University of Chicago Medical Center, explains the differences between an outbreak, an epidemic, and a pandemic, and answers questions about the swine flu outbreak.
Experts warn that a new flu pandemic could kill 20 million people globally. There have been multiple flu pandemics in the 20th century. The Spanish flu pandemic of 1918 is estimated to have caused 50 million deaths, with anywhere from 20% to 40% of the world's population falling sick. The Asian flu pandemic of 1958–59 resulted in the death of about 2 million people and the Hong Kong flu of 1968–69 is estimated to have caused around 1 million deaths.
The only effective mitigation against a pandemic is a vaccine. Scientific advances have considerably shortened the time it takes to develop a new vaccine. However, it is still estimated to be around 30 weeks. Experts believe that shortening this to 6 weeks is the only way to curb the spread of a pandemic.