Mostly, humans have only experienced infections by three different H types (H1, H2 and H3), and two different N types (N1 and N2).
Presently, two subtypes, H1N1 and H3N2, circulate among human beings, causing the seasonal flu epidemics.
Since these strains are well adapted to humans, they are referred to as human flu rather than bird flu.
Whenever a new flu A virus establishes itself in humans, it can cause a pandemic, and four such pandemics have occurred since 1918,
1918, the Spanish flu (H1N1)
The 1957-58 Asian flu (H2N2)
The 1968 Hong Kong flu (H3N2)
The 2009 swine flu (caused by a newer version of the H1N1).
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