Influenza or the flu, is a respiratory illness that affects more than one billion people worldwide each year. Vaccination for most is simply a precautionary measure to avoid the fever, sore throat or muscle aches that come with the flu. Seasonal flu is extremely dangerous for some: people whose immune systems have been weakened by age or illness. But it is not usually life threatening for most healthy people. Pandemic flu is another matter. Pandemic flu occurs when a new strain of influenza emerges that can be transmitted easily from person to person and for which there's little or no natural immunity. Unlike seasonal flu, most people have not built up resistance to it and pandemic flu can kill those who are young and healthy as well as those who are frail and sick.

In addition to its societal implications, influenza results in a substantial economic burden in the form of hospital and other health care costs and lost productivity. In the United States alone recent estimates put the cost of influenza epidemics to the economy at US$ 71-167 billion per year.