I disagree, YamotoTwinkie. IIRC, the USDA bases their recommended cooking temperatures on the heat needed to kill the potential human-contagious diseases that might be in the meat. Chicken is based on killing Salmonella species, pork is based on killing Trichinella. So, it’s more of a pathogen difference than a prevalence of pathogen issue.
<soapbox>The US has some of the safest meat in the world because there are complex, multilayer testing and monitoring procedures, from the farm level all the way up to the grocery store</soapbox>
In general, the more closely related you are to a species, the easier it is for you to catch a disease from it. Pigs have a number of piggie 'flu viruses that jump to humans. Beef vector human tapeworms and tuberculosis. Fish are probably the least catching. I’m only familiar with 2 diseases (an roundworm and a Mycobacterium) that we can catch from fish.
Setting animals Free-range does not guarantee that they have fewer diseases than confined animals. You can’t keep free-range animals from picking up diseases off of wild animals like you can when they are confined. And, free-ranging animals will still pass disease between one another like confined animals. But, when they are confined, you can eradicate a disease more easily to keep it from spreading to nearby herds, unlike free-ranging animals. It is nicer for the animal to be free-range, however, and it can be done successfully in limited situations with smaller groups of animals. But, feeding the human population the amount of meat it wants on the amount of land we have requires raising animals in very dense groups.