Influenza A(H1N1) virus is a subtype of influenzavirus A and the most common cause of influenza (flu) in humans. Some strains of H1N1 are endemic in humans and cause a small fraction of all influenza-like illness and a large fraction of all seasonal influenza. H1N1 strains caused roughly half of all human flu infections in 2006. Other strains of H1N1 are endemic in pigs (swine influenza) and in birds (avian influenza).
In June 2009, WHO declared that flu due to a new strain of swine-origin H1N1 was responsible for the 2009 flu pandemic. This strain is commonly called “swine flu” by the public media.
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 proteins found on the surface of the virus: hemagglutinin (H) and neuraminidase (N). All influenza A viruses contain hemagglutinin and neuraminidase, but the structure of these proteins is said to differ from strain to strain due to rapid genetic mutation in the viral genome. However, reconstruction of the genome through sequencing techniques such as RNA-Seq is a heuristic process because trying all possible alignments of the short reads is computationally prohibitive, therefore none of the possible alignments can be proven to be correct. This lack of experimental repeatability might lead to measuring noise being incorrectly interpreted as a mutation or a new virus.
Influenza A virus strains are assigned an H number and an N number based on which forms of these two proteins the strain contains. There are 16 H and 9 N subtypes known in birds, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans.
The flu pandemic Spanish flu , also known as La Gripe Española, or La Pesadilla, was an unusually severe and deadly Strain (biology)strain of avian influenza, a virus viral infectious disease, that killed some 50 million to 100 million people worldwide over about a year in 1918 and 1919. It is thought to be one of the most deadly pandemics in human history. It was caused by the H1N1 type of influenza
The Spanish flu caused an unusual number of deaths because it may have caused a cytokine storm in the body. ref cite journal author=Kobasa D, Jones SM, Shinya K, et al title=Aberrant innate immune response in lethal infection of macaques with the 1918 influenza virus journal=Nature volume=445 issue=7125 pages=319–23 year=2007 month=January pmid=17230189 doi=10.1038 nature05495 ref cite journal author=Kash JC, Tumpey TM, Proll SC, et al title Genomic analysis of increased host immune and cell death responses induced by 1918 influenza virus journal.The recent epidemic of bird flu, also an Influenza A virus, had a similar effect. journal author=Cheung CY, Poon LL, Lau AS, et al title=Induction of proinflammatory cytokines in human macrophages by influenza A (H5N1) viruses: a mechanism for the unusual severity of human disease? journal=Lancet volume=360 issue=9348 pages=1831–7 year=2002 month=December |pmid=12480361.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. This makes it difficult for the patient to breathe. 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.cite book|last=Barry|first=John M.|authorlink=John M. Barry|title=The Great Influenza: The Epic Story of the Greatest Plague in History
Reff : wikipedia.org