The WHO will convene an expert panel on April 29 to attempt to answer that question, but one way to begin is to look at where the virus originated.
Epidemiologists appear to be homing in on a possible ground zero in the Mexican Gulf Coast state of Veracruz, in a town called Perote, which is home to a large pig farm owned by the U.S. company Smithfield Foods. Flu-like cases began popping up there in early April, before the first confirmed case in Mexico on April 13.