Influenza A viruses have been recognised in North American pig populations since the 1918-1919 'Spanish flu' pandemic.  H1N1 viruses are the most common, although H3N2 viruses have also been reported. Because pigs can be infected by both avian and human strains, they are considered 'mixing vessels' for the generation of novel strains of reassortant influenza A viruses capable of spreading to non-immune human populations.
The pandemic that started in spring 2009 was caused by a novel H1N1 subtype of influenza A in North America. Swine influenza causes respiratory illness similar to seasonal influenza. Unlike avian influenza, swine influenza, upon infecting humans, is easily transmissible from person to person. The earliest cases in the pandemic occurred in Mexico, where severe pneumonia and high rates of hospital admissions and deaths were reported. 
The World Health Organization (WHO) declared the 2009 novel H1N1 swine influenza pandemic to be over in August 2010.  However, it is presumed that the H1N1 virus that emerged in 2009 will continue to circulate along with seasonal strains of influenza.  Although severe respiratory failure and fatalities continue to be reported in affected countries, most cases worldwide have been characterised by a relatively mild respiratory illness that resolves with or without medical treatment by 7 to 10 days.
Since July 2011, 12 cases of H3N2 swine-origin influenza have been reported to the CDC in the US. Most patients reported direct or indirect contact with swine, although several cases appeared to result from human-to-human transmission. The novel swine-origin H3N2 influenza A strain has been sequenced and found to contain 7 gene segments from swine H3N2 influenza viruses that have been circulating in North American swine populations since 1998, and 1 gene segment (M gene) derived from the 2009 H1N1 influenza A virus.  Most patients presented with influenza-like illness, and all have recovered fully. The current seasonal influenza vaccine is likely to confer limited protection against this swine influenza variant strain. [CDC: Novel influenza A (H3N2) virus transmission] [CDC: Prevention strategies for seasonal and influenza A(H3N2)v in health care settings]