Did factory farming cause the swine flu?

My article on the possible link between swine flu and industrial farming that I wrote for Macleans.ca is online:

No one yet knows for sure where this swine-flu came from, but the chance that pig manure spread an infection is “quite possible,” says Ellen Silbergeld, an epidemiologist and professor at the Johns Hopkins Bloomberg School of Public Health who has done studies on flies on poultry farms spreading pathogens.