Farming changed radically after the 1950s, so pretty recently. It's pretty reasonable to believe it will. If you've been anywhere near mega livestock operations of any kind then you would know.
Well certainly antibiotics resistance (MRSA [1]) is a problem exacerbated by intense farming practices [2]. To the best of my knowledge there are two big sources of MRSA: Hospitals and livestock farming (the latter of which actually got its own acronym LA-MRSA as in livestock associated MRSA).
Now don't get me wrong, I'm not trying to directly compare the development of bacterial antibiotics resistance with the adaptation of viruses to be able to infect other kinds of hosts. Surely these disease vectors follow different developments.
It wouldn't surprise me though if intensive animal farming [3] has the capacity to exacerbate these problems, if only based on the high concentration of animals kept together and the generally poor health of these animals (poorly functioning immune systems, which is the whole reason for the overuse of antibiotics).
