mixing milk dramatically increases these chances.

If each person drinks milk from one cow, and there are 1 million of each, then you have 1 million chances.

A chance is higher if a cow has a few mutations in its viruses, or if a person has a slightly different receptor or immune system...

But if you put all the milk from the 1 million cows in a mixing vat and then distribute it to the same 1 million people, now there are 1 trillion chances, and the slightly better mutations will end up coming into contact with the humans with slightly more receptive cells.

This is one of many reasons I think we should limit the fanin/fanout of food production. Ie. you may only mix animal products in say batches of up to 10,000 cows, and that batch may only be split up amongst up to 10,000 humans. Then you have to clean the production equipment before the next batch.