H1N1

Overview
H1N1 is a subtype of the species Influenza A virus. H1N1 has mutated into various strains including the Spanish Flu strain (now extinct in the wild), mild human flu strains, endemic pig strains, and various strains found in birds.

A variant of H1N1 was responsible for the Spanish flu pandemic that killed some 50 million to 100 million people worldwide over about a year in 1918 and 1919. A different variant exists in pig populations. Controversy arose in October 2005, after the H1N1 genome was published in the journal Science. Many fear that this information could be used for bioterrorism.

"When he compared the 1918 virus with today's human flu viruses, Dr. Taubenberger noticed that it had alterations in just 25 to 30 of the virus's 4,400 amino acids. Those few changes turned a bird virus into a killer that could spread from person to person."

Low pathogenic H1N1 strains still exist in the wild today, causing roughly half of all flu infections in 2006.

Spanish Flu
The Spanish Flu, also known as La Grippe, or La Pesadilla, was an unusually severe and deadly strain of avian influenza, a 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 have been one of the most deadly pandemics so far in human history. It was caused by the H1N1 type of influenza virus, which is similar to bird flu of today, mainly H5N1 and H5N2.

The Spanish flu caused an unusual number of deaths because it, like H5N1, caused a cytokine storm in the body. The virus infected lung cells, leading to overstimulation of the immune system via release of cytokine bursts into the lung tissue. This leads to extensive leukocyte migration towards the lungs, causing destruction of lung tissue and secretion of liquid into the lung, and making it difficult for the patient to breathe. Due to the nature of the infection, people with a normal healthy immune system were more susceptible to the disease, such as young adults compared to young children and the elderly.

Russian flu
The 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. Some have called it a flu pandemic but because it only affected the young it is not considered a true pandemic. The virus was included in the 1978-1979 flu vaccine.