Both influenza A and B viruses are important respiratory pathogens, although influenza A viruses are the main cause of large epidemics with high mortality. Influenza occurs all over the world, with an annual global attack rate estimated at 5-10% in adults and 20-30% in children. Whereas in temperate climates outbreaks are experienced mainly during the winter season, influenza occurs more unpredictably in tropical regions. Recorded since the middle of the 18th century, new influenza A subtypes have caused major global outbreaks at unpredictable intervals. Of these pandemics, the "Spanish flu" in 1918 was the most severe, causing an estimated 20-40 million or more deaths worldwide. less severe pandemics occurred in 1957 and 1968.
Precise data on influenza morbidity and mortality are available mainly from industrialized countries. Seasonal increases of morbidity and mortality above a predicted baseline are frequently used as a measure of the relative serverity of influenza epidemics. In the United States, the average excess hospitalization associated with influenza in infants less than 6 months of age was found to approach 1000 per 100 000. For previously health children of less than 4 years of age, the corresponding average rate was 100 per 100 000 and in the age group of 5-15 years, 40 per 100 000. For children at particular risk of serious infection, these annual rates were about 5 times higher than in previously healthy individuals.