HISTORY

Periods of respiratory disease have been recorded to come and go from the time of Hippocrates to the present. However, it is unknown if the cases before the twentieth century were actually caused by influenza viruses. Seroarcheaological evidence of influenza only dates back to 1889 because such tests require blood samples and blood storage only began in the 1970s. In 1933, the first influenza virus was isolated. A serious strain in Hong Kong right now is H5N1 ("bird flu").

Here is a list of the major antigenic shifts of Influenza A.

Year	HA and NA genes
1989	H2N2
1900	H3N8
1918	H1N1 "Spanish"
1957	H2N2 "Asian"
1968	H3N2 "Hong Kong" *
1977	H1N1 "Russian"

* = probably current within human populations.

Beyond the genetic history of the virus, influenza has a prominent place in 20th century human history. During the 1918-1919 pandemic, Influenza A was responsible for 20-40 million deaths, and played a major role in World War 1. PBS recently had a documentary on the 1918 pandemic.
For a detailed history of the pandemic, look at Molly Billings 1918-1919 Pandemic Page.