A lot of it has to do with the factory farming methods. When you force animals to live in such crowded, sometimes squalid, conditions you invite diseases to breed and grow. When animals are pasture-raised or free-ranged and then properly rotated/managed, the sun, one of the strongest disinfectants we have, is able to help clean out the bacteria and virii. Not to mention that the animals are far less stressed and therefore have stronger immune systems! Our factory-farm complex is trying to breed pigs that don't experience stress so they can cram them in together rather than finding ways to make proper farm management more affordable and common.