Whether they're north or south of the border,factory farms are a breeding ground for disease.

A quick show of hands: how many people think that this recombinant H1N1 (swine) flu emerged from a factory farm?

OK, thank you. You can put your hands down and go wash them again.

Of course, even for people (like me) who think that this new virus probably incubated within massive pig farms -- excuse me, a concentrated animal feeding operations (CAFOs) -- there still isn't clear evidence of exactly how or where the virus emerged.

There is, however, some indication that Mexico might not be country of origin for the new virus (at least, not the swine flu component). Recent reporting points to the the existence of the swine flu strain of this new virus in 1998 among pigs bred in the United States.

And about these farms, a.k.a. feeding operations or containments: how does disease spread in them?

First, they're crowded. A single containment might hold as many as 10,000 pigs. Containments are usually grouped together over several acres. The facility in La Gloria, Mexico, has been reported to house almost one million swine. As is typical for such operations, they are sealed up to prevent external pathogens from infecting and then decimating the thousands of immune-compromised swine inside the building.

Pigs (or poultry, for that matter) are packed into standing-room-only cages and injected with antivirals and antibiotics to keep them alive in their cramped, squalid pens until they're large enough for slaughter.

The second point is that factory farms are extremely dirty. The Environmental Protection Agency estimates that half of the 500 million tons of manure produced by livestock comes from concentrated feeding operations operations. "The 500,000 pigs at a single Smithfield subsidiary in Utah generate more fecal matter each year than the 1.5 million inhabitants of Manhattan," as reported in Rolling Stone in 2006.

This semi-liquid waste is a soil, air and water pollutant. The pathogens shed in livestock waste can remain viable for months if left untreated. This can increase the chance of viral transmission to insects or birds, which could then reinfect other wildlife or captive livestock.

The workers who transport and treat this waste, or those who corral and slaughter the animals, are also at risk for infection. Those workers, their families and their neighbors can then become potential agents of disease transmission, especially when the workers are not provided with protective clothing, boots or headgear (even when the use of such equipment is mandated).

(Additionally, these facilities are often located in close proximity to poor communities, or, in this country, those that are poor, rural and/or mostly comprised of minority populations. Studies have shown higher incidence of respiratory and immunological disorders among residents of some of these neighboring communities.)

Last year, the Union of Concerned Scientist conducted a study that addressed the economic, environmental and heath consequences of expanded animal-containment operations. The group asserted that billions of taxpayer dollars essentially subsidized factory farms and their methods. UCS recommends using that money to develop and promote more beneficial and sustainable livestock-rearing methods.