Swine flu is an influenza, a virus particular to pigs, but common to all mammals and birds. There are many different strains of influenza, and once they pass to the human population they are coded, catalogued, and studied.

Common characteristics of the influenza viruses include incubation periods of varying degrees, symptoms that include coughing and sneezing, high fever, muscle pains, sore throat, weakness and fatigue as well as debilitating headaches. All are transmitted the same way - through human social contact in the form of airborne body fluids - through coughing, sneezing, and generally projecting the virus from one person to the next.

The "Great Influenza" of 1918 is recognized as the worst in modern times, killing an estimated 50 millions across the globe in just a single year. The virus strain was a virulent and cruel version called H151, or more infamously, the "Spanish Flu". Most now believe that it started during the closing months of World War 1, and was rapidly transmitted by the close and squalid ranks of millions in trenches across Europe. It crossed to America in troop ships, ravaged camps of densely packed new recruits in the heartland, and engulfed America from there.

The same H151 strain from 1918 has now returned, in a yet unknown form as Mexican Swine Flu.

The following video was produced by the Johns Hopkins Centre for Civilian Biodefence Studies. It was developed to answer the question "what would happen in a bio-weapon attack on America?", but uses the backdrop of the 1918 influenza as a guide. Notable are the early newspaper headlines from the period, which echo those of today and the conclusion, which is not that settling at all

The most comprehensive - and readable - book on The Great Influenza remains The Great Influenza: The story of the deadliest pandemic in history by john M Barry. In it, Barry traces the cause through the pandemic to its end in a day by day account of what really was a very short span of time. The book remains as good a blueprint for the future as anything else we have today, once the pandemic takes hold.