How did the whole swine flu thing start?

The 2009 H1N1 pandemic, commonly known as swine flu, was caused by a novel influenza A (H1N1) virus that emerged in Mexico and the United States in early 2009.

Before that, human influenza A viruses were known to circulate in three groups, known as A, B, and C. Human influenza A viruses are further divided into subtypes based on the combination of hemagglutinin (H) and neuraminidase (N) proteins. Before 2009, seasonal influenza viruses belonged to any of two subtypes: H1N1 or H3N2.

The 2009 pandemic virus, referred to as A(H1N1)pdm09, shares genetic material from different swine flu viruses, human flu viruses, and avian flu viruses. The precise origins of the A(H1N1)pdm09 virus remain unclear, but it is believed to be a result of genetic reassortment between swine influenza A viruses circulating in pigs and human seasonal influenza A viruses.

Specifically, the A(H1N1)pdm09 virus contains genes from swine influenza A viruses, Eurasian avian-like swine viruses, and human-adapted influenza A viruses. Genetic sequencing of the 2009 H1N1 virus provided evidence of "triple reassortment," involving gene segments from North American swine influenza viruses (H1N2), Eurasian swine influenza viruses (H1N1), and influenza viruses that regularly cause seasonal epidemics in humans (H3N2).

In summary, the 2009 swine flu pandemic was caused by a novel influenza A (H1N1) virus that emerged through genetic reassortment between different strains of swine influenza viruses, avian flu viruses, and human seasonal influenza viruses.