History of Influenza

Spanish Flu (1918–1920)
The 1918 flu pandemic, commonly referred to as the Spanish flu, was a category 5 influenza pandemic caused by an unusually severe and deadly Influenza A virus strain of subtype H1N1.
The difference between the influenza mortality age-distributions of the 1918 epidemic and normal epidemics. Deaths per 100,000 persons in each age group, United States, for the interpandemic years 1911–1917 (dashed line) and the pandemic year 1918 (solid line).
The Spanish flu pandemic lasted from 1918 to 1919, although Price-Smith's data suggest it may have begun in Austria in the Spring of 1917. Older estimates say it killed 40–50 million people while current estimates say 50 million to 100 million people worldwide were killed. This pandemic has been described as "the greatest medical holocaust in history" and may have killed as many people as the Black Death, although the Black Death is estimated to have killed over a fifth of the world's population at the time, a significantly higher proportion. This huge death toll was caused by an extremely high infection rate of up to 50% and the extreme severity of the symptoms, suspected to be caused by cytokine storms. Indeed, symptoms in 1918 were so unusual that initially influenza was misdiagnosed as dengue, cholera, or typhoid. One observer wrote, "One of the most striking of the complications was hemorrhage from mucous membranes, especially from the nose, stomach, and intestine. Bleeding from the ears and petechial hemorrhages in the skin also occurred." The majority of deaths were from bacterial pneumonia, a secondary infection caused by influenza, but the virus also killed people directly, causing massive hemorrhages and edema in the lung.
The Spanish flu pandemic was truly global, spreading even to the Arctic and remote Pacific islands. The unusually severe disease killed between 2 and 20% of those infected, as opposed to the more usual flu epidemic mortality rate of 0.1%.Another unusual feature of this pandemic was that it mostly killed young adults, with 99% of pandemic influenza deaths occurring in people under 65, and more than half in young adults 20 to 40 years old. This is unusual since influenza is normally most deadly to the very young (under age 2) and the very old (over age 70). The total mortality of the 1918–1919 pandemic is not known, but it is estimated that up to 1% of the world's population was killed. As many as 25 million may have been killed in the first 25 weeks; in contrast, HIV/AIDS has killed 25 million in its first 25 years.

The Manchester Influenza Epidemic of 1937
The Epidemic that never escalated to Pandemic This is an example of an interwar epidemic that public health controls did not allow to develop into a full blown pandemic because of what was already known from 1918, as well as employing very strict patient, contact and family isolation. In 1937 there were 620 claims for sickness benefits made to various insurance companies.

Asian Flu (1957–1958)
The "Asian Flu" was a category 2 flu pandemic outbreak of avian influenza that originated in China in early 1956 lasting until 1958. It originated from mutation in wild ducks combining with a pre-existing human strain. The virus was first identified in Guizhou. It spread toSingapore in February 1957, reached Hong Kong by April, and US by June. Death toll in the US was approximately 69,800. The elderly were particularly vulnerable Estimates of worldwide deaths vary widely depending on source, ranging from 1 million to 4 million.

Hong Kong Flu (1968–1969)
The Hong Kong Flu was a category 2 flu pandemic caused by a strain of H3N2 descended from H2N2 by antigenic shift, in which genes from multiple subtypes reassorted to form a new virus. The Hong Kong Flu pandemic of 1968 and 1969 killed an estimated one million people worldwide. Those over 65 had the greatest death rates. In the US, there were about 33,800 deaths.

2009 Flu Pandemic (Since 2009)
In March-April 2009, an epidemic of influenza-like illness of unknown causation occurred in Mexico. On April 24, 2009, following the isolation of an A/H1N1 influenza in 7 ill patients in the southwest US. The WHO issued a statement on the outbreak of "influenza like illness" in the confirmed cases of A/H1N1 influenza had been reported in Mexico, and that 20 confirmed cases of the disease had been reported in the US.The next day, the number of confirmed cases rose to 40 in the US, 26 in Mexico, 6 in Canada, and 1 in Spain.The disease spread rapidly through the rest the spring, and by May 3, 787 confirmed cases of the disease had been reported worldwide. On June 11, 2009, the ongoing outbreak of Influenza A/H1N1, commonly referred to as "swine flu", was officially declared by the WHO to be the first influenza pandemic of the 21st century and a new strain of Influenza A virus subtype H1N1 first identified in April 2009. It is thought to be amutation (reassortment) of four known strains of influenza A virus subtype H1N1: one endemic in humans, one endemic in birds, and two endemic in pigs (swine).
In November 1, 2009 a worldwide update by the U.N.'s World Health Organization (WHO) stated that "199 countries and overseas territories/communities have officially reported a total of over 482,300 laboratory confirmed cases of the influenza pandemic H1N1 infection, that included 6,071 deaths.