It's not that cut and dry. I think contagiousness is the #1 factor here.

What I've not seen mentioned thus far is whether the "non-sick" pax has any medical issues which might make such an arrangement a health problem.

To make a long story short, I was quite sick in 2013. After seeing countless doctors who made things worse, I finally got ahold of one who decided it was best to "reboot" my immune system. They essentially nuked it and started over. Worked wonderfully...with one small problem -- I couldn't take antibiotics for 12-18 months. You better believe I was washing my hands at every opportunity, wearing a mask when a room/plane sounded more like a TB ward.

The person with GI distress due to food poisoning wouldn't have been a problem for me. BUT If the person was having GI distress due to something contagious (like Clostridium difficile), my health would have been a major concern.

At the same time I also have sympathy for those who are traveling whilst sick -- I battled C.Diff for a year and had to keep working (which meant traveling).

I don't see why one's "medical history" in this context is a privacy issue -- it's no different than sitting next to someone with a peanut allergy. The actions of one person could gravely affect another pax on the plane, thus there needs to be some disclosure. It doesn't need to be detailed or overly-intrusive.