(source from wikipedia: http://en.wikipedia.org/wiki/2009_swine_flu_outbreak)
The outbreak is believed to have begun in March 2009. Local outbreaks of an influenza-like illness were first detected in three areas of Mexico, but the virus responsible was not clinically identified as a new strain until April 24, 2009. Its presence was soon confirmed in various Mexican states and Mexico City.
Within days isolated cases (and suspected cases) began to appear elsewhere in Mexico, the U.S., and several other Northern Hemisphere countries.
By April 28, the new strain was confirmed to have spread to Spain, the United Kingdom, New Zealand, and Israel, and the virus was suspected in many other nations, including South Korea and Austria, with over 3,000 candidate cases, prompting the World Health Organization (WHO) to raise its pandemic alert level to "phase 5".