Why is the exact origin of the 2009 H1N1 virus uncertain?

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Caring for flu victims — influenza pandemic (H1N1) of 2009 Doctors at the Mexico City Navy Hospital wearing protective gear as they tend to patients complaining of H1N1 flu-like symptoms.

The exact origin of the 2009 H1N1 virus remains uncertain because it evolved through a complex process known as genetic reassortment.12

While the virus is often referred to as swine flu due to the suspicion that it originated in pigs, this remains a point of speculation. The virus contained a unique combination of genetic material derived from three distinct sources: two strains of swine influenza, as well as human and avian influenza viruses.12

Researchers believe these different viruses presumably infected the same host—likely a pig—simultaneously, allowing them to exchange genetic material and create the pandemic strain. However, the specific details regarding how and when this reassortment occurred have not been definitively established.12