H1n1 Flu

Some strains of H1N1 are endemic in humans, including the strain(s) responsible for the 1918 flu pandemic which killed 50–100 million people worldwide. Less virulent H1N1 strains still exist in the wild today, worldwide, causing a small fraction of all influenza-like illness and a large fraction of all seasonal influenza.

H1N1 strains caused roughly half of all flu infections in 2006. Other strains of H1N1 are endemic in pigs and in birds.
In March and April 2009, hundreds of laboratory-confirmed infections and a number of deaths were caused by an outbreak of a new strain of H1N1.


Nomenclature


The various types of influenza viruses in humans. Solid squares show the appearance of a new strain, causing recurring influenza pandemics.Broken lines indicate uncertain strain identifications.

Influenza A virus strains are categorized according to two viral proteins, hemagglutinin (H) and neuraminidase (N). All influenza A viruses contain hemagglutinin and neuraminidase, but the structure of these proteins differs from strain to strain due to rapid genetic mutation in the viral genome.

It is thought to be one of the most deadly pandemics in human history. It was caused by the H1N1 type of influenza virus.
The Spanish flu caused an unusual number of deaths because it may have caused a cytokine storm in the body. (The recent epidemic of bird flu, also an Influenza A virus, had a similar effect.) The Spanish flu virus infected lung cells, leading to overstimulation of the immune system via release of cytokines into the lung tissue.This leads to extensive leukocyte migration towards the lungs, causing destruction of lung tissue and secretion of liquid into the organ. In contrast to other pandemics, which mostly kill the old and the very young, the 1918 pandemic killed unusual numbers of young adults, which may have been due to their healthy immune systems being able to mount a very strong and damaging response to the infection.
The term "Spanish" flu was coined because Spain was at the time the only European country where the press were printing reports of the outbreak, which had killed thousands in the armies fighting the First World War.

Other countries suppressed the news in order to protect morale.
Russian flu
See Influenza A virus subtype H2N2#Russian flu for the 1889–1890 Russian flu
The more recent Russian flu was a 1977–1978 flu epidemic caused by strain Influenza A/USSR/90/77 (H1N1). It infected mostly children and young adults under 23 because a similar strain was prevalent in 1947–57, causing most adults to have substantial immunity.However, as the virus has not yet been isolated in animals to date and also for historical naming reasons, the World Organisation for Animal Health (OIE) suggests it be called "North-American influenza". On April 30, 2009 the World Health Organization began referring to the outbreak as "Influenza A (H1N1)" instead of "swine flu", presumably to countermand the consequences of the misleading less scientifically labeled name as referenced by WHO spokesperson Dick Thompson.
Several complete genome sequences for U.S. flu cases were rapidly made available through the Global Initiative on Sharing Avian Influenza Data (GISAID). Preliminary genetic characterization found that the hemagglutinin (HA) gene was similar to that of swine flu viruses present in U.S.

pigs since 1999, but the neuraminidase (NA) and matrix protein (M) genes resembled versions present in European swine flu isolates. "Induction of proinflammatory cytokines in human macrophages by influenza A (H5N1) viruses: a mechanism for the unusual severity of human disease?".