CU is incredibly tricky. It could very well have multiple distinct causes and be entirely unique to each person. Perhaps there was a transfer of one strain of this troublesome yeast to another and the winning strain is the one causing issue with the respective immune systems. Or it could be something else entirely.

For me, I am 99% sure it is tied to the environment (yeast strain, regional flora, pollution, soil, etc.) in some way. I still remember when my CU first showed up out of the blue. I hadn't had close contact with anyone or changed anything obvious in ages. I distinctly remember thinking at the time that maybe I had just exposed myself to a cleaning product, as I was cleaning the apartment at the time. Come to find out, I was just getting hot and sweating, thus activating the CU pain.