Other blogsters have latched onto one possibility for a flu virus vector - flies that love manure lagoons near Mexican CAFOs. But they seem honest about the fact this is only an hypothesis at this point.
Pigs ARE prodigious defecators.
A vaguely related statistic to ponder from Stuffed and Starved.
In the US feedlots produce 300 million tons of manure a year.
As I recall this is about bovines, though maybe all feedlots and CAFOs - but whatever - that's a ton of s--- per person. Are our meat and dairy habits worth it?