Pandemic 2009 h1n1 influenza (swine flu)

[Zipporah Schumaker]

Pandemic 2009 H1N1 influenza is caused by a strain of H1N1 influenza A virus that genetically is a combination of swine, avian, and human influenza viruses.
(See also Influenza.)
Most often, pigs have been infected by strains of influenza that are slightly different from those that infect people. These strains very rarely spread to people, and when they do, they very rarely then spread from person to person.
The novel influenza H1N1 virus, A(H1N1)pmd09, initially referred to as H1N1 swine flu virus, is a combination of swine, avian, and human influenza viruses that spreads easily from person to person. The infection is not acquired through ingestion of pork and is acquired very rarely by contact with infected pigs.
In June 2009, the World Health Organization declared H1N1 swine flu a pandemic. It spread to gt; 70 countries and to all 50 US states. The majority of the deaths initially occurred in Mexico. The attack rate and mortality for H1N1 swine flu are higher in young and middle-aged adults and lower in older patients than they are for seasonal flu, possibly because younger people lack prior exposure to similar influenza viruses. The pandemic entered the postpandemic period in August 2010. Subsequently, the virus name was standardized to influenza A(H1N1)pdm09 to denote the pandemic and distinguish the virus from seasonal H1N1 strains and the 1918 pandemic H1N1 strain. Since 2009, influenza A(H1N1)pdm09 has circulated as a seasonal influenza.
When humans are infected with influenza strains that more closely resemble those that infect pigs, the strains are called swine variant viruses and denoted with a quot;vquot; (eg, H3N2v). Over 400 human cases of H3N2v and several cases of H1N1v and H1N2v infections have occurred sporadically in several US states where children and adults have had contact with both sick and apparently healthy domestic pigs at agricultural fairs. There have also been cases of possible human-to-human transmission. The H3N2v virus has genes from avian, swine, and human viruses and the matrix (M) gene from the A(H1N1)pdm09 virus; most cases occurred in the summer of 2012.