Words like epidemic and pandemic are tossed around frequently, especially with the media hype surrounding a potential avian bird flu and with the spread of the H1N1 "swine flu". However, there is a very concrete definition of a pandemic, as defined by major organizations and specialists in the field. Specifically, a pandemic is an epidemic of an infectious disease that spreads through human populations across a vast region, generally a continent or even worldwide.
An epidemic is any time a particular disease spreads at a rate far greater than expected. Examples of epidemics include cancer, heart disease, H1N1 and the infamous black plague. However, not all of these are pandemics. Another criterion for a pandemic is that the disease be highly infectious, which immediately roles out non-infectious diseases like heart disease and cancer.
The World Health Organization (WHO) has three primary criteria to define a pandemic. First, the disease has to spread into new populations. Secondly, it must be an infectious agent that causes serious illness in humans. Lastly, it must spread efficiently, sustainably, and quickly.
Antibiotic or antiviral drug resistance is a major concern associated with pandemics. Because of their high infectious rates, these diseases can spread and mutate quickly, making it difficult for drug manufacturers to develop adequate vaccines or treatments. Malaria, for instance, is quickly developing powerful resistance to anti-malarial drugs, making our efforts to fight its spread nearly worthless.
There are many recorded instances of pandemics.
1) H1N1 "swine flu" - The most recent pandemic is the worldwide spread of the H1N1 flu. The WHO raised its pandemic alert level to the highest possible for the first time in more than 40 years. As of the summer of 2009, more than 30,000 cases had been confirmed worldwide, certifying H1N1's status as a pandemic.
2) 1918 Spanish Flu. This strain of H1N1 infected people on inhabited continents and saw nearly 500 million total infections. It's estimated that between 40 and 80 million died from this particularly deadly strain of influenza.
3) Smallpox. In the 20th century, smallpox was estimated to have killed nearly 400 million worldwide. It was officially declared eradicated in 1979.
4) Malaria. Malaria is prevalent in parts of the Americas, Asia and Africa, with about 400 million cases worldwide.
5) Measles. A very contagious disease, measles spread so quickly that nearly 90% of all people were infected by the age of 16. However, after a vaccine was introduced, the number of cases has dropped considerably. It is still a major killer, with 197,000 people dying each year to measles worldwide.