>But is it really necessary to refer to such infections being "physiologically stressful"? I just don't see the relevance of that.

Viral infections activate the HPA axis...that is what is meant by "physiologically stressful". See:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1224723/

The relevance is that stress seems to be the common factor as a trigger, and the stress system seems to be dysfunctional in CFS.