The pattern of influenza epidemics is shown diagrammatically in Fig. 1. In a seven-year period, which can be any seven-year period during the past 50 years, epidemics of influenza have been recorded in all countries where monitoring of infection has been studied. The diagram shows that during the winter months there is an excess of deaths in the years when influenza is not recorded. In epidemic years, 10% or more of a population may be infected; 50% of infected persons will develop symptoms, and an excess number of deaths will occur (Christie 1987). The number of deaths is proportional to the number of people infected, usually about 0·1%, and influenza has been described as an invariable disease caused by a variable virus. Most of the deaths occur among the elderly, such as those with chronic heart and lung disease or metabolic disorders. Although vaccine against the prevalent virus strains is produced each year, the use of this has not been wide: only a proportion of the ‘at risk’ group receive vaccine, and although shown to protect 60–90% of the individuals against infection, and a higher percentage against hospitalization, immunization has never made an impact on the course of an epidemic.