Influenza is a highly contagious acute respiratory disease caused by an RNA virus of the orthomyxoviridae family. The name influenza comes from the old medical belief in unfavourable astrological influences as the cause of the disease. Influenza is a major cause of illness and causes suffering in all age groups.

Epidemics of influenza typically occur during the winter months in temperate regions. In annual influenza epidemics, according to the World Health Organization, 5-15 percent of the population is affected with upper respiratory tract infections. Every year there are 3-5 million cases of severe illness and between 250,000 and 500,000 deaths around the world due to influenza. In the Northern Hemisphere, 100 million people are infected with the influenza virus each year and most of these patients are debilitated in some way.

Influenza has been responsible for an average of approximately 36,000 deaths/year in the United States during 1990 - 1999. In the UK about 6,000 excess deaths occurred at the time of influenza outbreaks during the years 1987 - 1995. The attack rate for influenza is highest among children aged 5-9 years, reaching 30% of this segment of the population. Although the illness burden in children is very high, the mortality due to influenza generally affects adults of 65 years or older.

Every 20 years or so, there is a major change in an influenza virus strain. When this major change occurs a worldwide epidemic, called a pandemic, almost inevitably follows because influenza spreads very quickly, especially with modern transportation methods. Pandemic influenza can produce an enormous increase in the annual death toll; the 1918 pandemic may have caused in excess of 40 million deaths worldwide.