Influenza A virus subtype H1N1

Influenza A virus subtype H1N1, commonly known as the swine flu virus, is the subtype of influenza A virus that caused the H1N1 flu pandemic in 2009. An earlier H1N1 variant is also believed to have caused the 1918-1919 pandemic known as the Spanish flu.

H1N1 2009 pandemic
Influenza A subtype H1N1 strains caused a small percentage of all human flu infections in 2004–2005. In June 2009, the World Health Organization (WHO) declared a new strain of swine origin as a pandemic, naming it A/H1N1pdm09. It spread worldwide and had caused about 17,000 deaths by the start of 2010. The H1N1 influenza pandemic was declared over on August 10, 2010. This variant of H1N1 is now a strain found in the usual seasonal flu group. Many people now have some level of immunity to this flu virus, so it is no longer considered as much of a concern as it was during 2009-2010. H1N1 protection has been included in the annual flu vaccines for a number of years now.

Spanish flu
The H1N1 variant that caused the 1918-1919 "Spanish flu" pandemic was also believed to have come from swine (pigs), before mutating to infect humans.

Other swine flu viruses
Other strains of inflenza A that evolved from swine include:
 * Influenza A virus subtype H1N2
 * Influenza A virus subtype H3N2

Influenza vaccines
Annual influenza vaccines protect against different influenza A subtypes including H1N1 and H3N2, and both groups of inflenza B.

ME/CFS
"Pandemic influenza A (H1N1) infection was associated with a more than two-fold increased risk of CFS/ME. We found no indication of increased risk of CFS/ME after vaccination. Our findings are consistent with a model whereby symptomatic infection, rather than antigenic stimulation may trigger CFS/ME."
 * 2015, Chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is associated with pandemic influenza infection, but not with an adjuvanted pandemic influenza vaccine.

Learn more

 * Swine flu - NHS
 * Types of Inflenza - CDC