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Annual influenza epidemics are estimated to affect 5 to 15% of the global population. Although most cases are mild, this still causes severe illness in 3 to 5 million people and around 250,000 to 500,000 deaths worldwide. In industrialized countries severe illness and deaths occur mainly in the high-risk populations of infants, the elderly, and chronically ill patients.

In addition to these annual epidemics, Influenza A virus strains caused three major global epidemics during the 20th century: the Spanish flu in 1918, Asian flu in 1957 and Hong Kong flu in 1968-69. These pandemics were caused by strains of Influenza A virus that had undergone major genetic changes and for which the population did not possess significant immunity. The overall effects of these pandemics and epidemics are summarized in the table below.

20th century flu pandemics
Pandemic	Year	Influenza A virus subtype	People infected (approx)	Deaths (est.)	Case fatality rate
1918 flu pandemic	1918-19	H1N1	0.5 to 1 billion (near 50%)	20 to 100 million	>2.5%
Asian flu	1956-58	H2N2		2 million	<0.1%
Hong Kong flu	1968-69	H3N2		1 million	<0.1%
Seasonal flu	Every year	mainly A/H3N2, A/H1N1, and B	5-15% (340 million to 1 billion)	250,000 - 500,000 per year	<0.05%