The example I used was the walk I took to the laundry mat, where we talked (8 minutes each way and doing my laundry). I told him I would not be able to do anything like that again for 3 days. He just said "ah man" with empathy, he really got it!
I basically told the truth. My immune system is hyper-active, similar to autoimmunity but without any specific organ being targeted.
I haven't had a cold or flu in 15-20 years or more, and my worst symptoms are flu-like, because my immune system is constantly over-active. Physical activity just increases my immune system activation, increasing my symptoms and causing PEM.