A pandemic occurs when a disease spreads over an extensive geographical area and affects a large proportion of people or animals.
It is an epidemic across a broad global area - hence the pan prefix - and usually, but not always, originates in an animal, from which it crosses over into humans who possess no pre-existing immunity to combat the disease.
Pandemics occur three or four times a century. Historically, the most famous is the Black Death, which occurred in Asia, the Middle East and - between 1347 and 1350 - Europe, leaving one third of the population dead.
The plague reappeared several times in Europe between the 14th and 17th centuries.
There were three major pandemics in the 20th century:
1918-19
The great influenza, also known as Spanish flu, was the most severe pandemic in history, with 20-40% of the world's population estimated to have suffered from it. Symptoms emerged that resulted in many people dying within 24 hours of their surfacing.
An unusual feature of the illness was that mortality rates were high among healthy adults between 25 and 40 years of age, as well as among high-risk groups such as the young and elderly. It killed more people than the first world war, with deaths numbering between 20-50m.
1957-58
Asian flu originated in the Asia Pacific region, spreading as far as North America. Improvements in science and technology resulted in rapid identification, but around one million died.
1968-69
Hong Kong flu was the least lethal of the major pandemics of the last century, killing 750,000. The influenza virus associated with the sickness still circulates today.
Aids
Many have described Aids to be a pandemic, but others are reluctant to use that description.
Professor Ian Jones, a biologist and avian flu expert at Reading University, considers it to be a major epidemic in that the illness has spread within sections of the world population rather than across the world and compares it to local but large flare-ups.
UNAids, the UN programme on HIV/Aids, also refers to the disease as being in an epidemic form.
