Homo Sapiens first evolved some 300,000 years ago, yet crowd infections are believed to have only developed in the last 12,000 years, a small blip in human history. Humans living in dense cities is a relatively recent development.

Seems to me that a big "elephant in the room" about this is animal agriculture, which also evolved in this "small blip" of human history and has clearly been a major factor these "crowd" diseases have leveraged along with air travel etc.

If vaccines etc are necessary to compensate for our immune system's short comings to modern problems, it seems reasonable that scaling down animal farming and consumption is also a necessary compensatory mechanism.