March 2009: The outbreak was probably detected in Mexico City first, where surveillance began picking up a surge in cases of influenza-like illness (3).
March-early April 2009: A new swine-origin A/H1N1 influenza virus emerged and spread rapidly in Mexico and the United States (10).
May 2009: 41 countries officially reported 11,034 human cases of H1N1 (3).
June 2009: The World Health Organization (WHO) declared a phase-6 pandemic alert, indicating a global pandemic (10).
2009-2010: Prevention and vaccination efforts increase worldwide.
July 2010: WHO announces that the world is no longer in a pandemic (10).
Here are some other historical influenza pandemics, caused by avian-origin viruses:
1918: known as the "Spanish Flu"; caused by an H1N1 virus of avian origin; 500 million people (1/3 of world population) became infected with the virus; most severe in recent history; led to 50 million deaths worldwide (8).
1957: a new influenza A H2N2 triggered the "Asian Flu" pandemic; comprised of three different genes from an H2N2 virus that originated from an avian influenza A virus; led to over 1 million deaths worldwide (8).
1968: caused by an influenza A (H3N2) virus comprised of two genes from an avian influenza A virus; led to over 1 million deaths worldwide; H3N2 virus continues to circulate worldwide as a seasonal influenza A virus (8).
The following short video was published by the Discovery Channel in 2009. It explains the how the H1N1 swine flu originated, the virus structure, and how if affects the human body.