article in The Guardian includes:

iseases have spread from wildlife to humans throughout history but we now interact with animals in a very different way, says Danielle Nierenberg, a researcher with the US Worldwatch Institute. “In the last 40 years the world has gone through a livestock revolution, not unlike what happened to crops with the green revolution,” she says.

Since 1961, she explains, worldwide livestock has increased 38%, to about 4.3 billion today. The global poultry population has quadrupled in that time, to 17.8 billion birds, and the number of pigs has roughly trebled to 2 billion. As the numbers of animals bred for food have vastly grown in a very short period, humankind’s relationship with them has changed.

“Raising animals has morphed into an industrial endeavour that bears little relation to landscape or natural tendencies of the animals. Wherever [industrial farming] is introduced it creates ecological and public health disasters,” she says.

Others argue that intensive confinement of animals promotes emerging viruses, stokes the development of antibiotic-resistant strains of bacteria and can transform animals into disease “factories”. According to Hans-Gerhard Wagner, an officer of the UN Food and Agriculture Organisation based in Thailand, the “intensive industrial farming of livestock is now an opportunity for emerging diseases”.