Slightly-off-topic:
This article presents the hard science as exceedingly difficult. I kinda suspect part of that is scientists have this attitude of acting completely blind to the hundreds of millions of years of evolution that made us what we are.
For example, if we suppose that microbiome is important to health, and suppose that all human behaviors (e.g. hand-shaking, kissing) I think it gets us pretty quickly toward some great avenues to test. I almost wonder if the imperfect sanitation conditions we evolved in might have conferred some ability to exchange fecal microbes.
Now obviously this doesn't really solve the fact that sequencing a microbiome is incredibly hard. But even without sequencing a microbiome you could probably do a blind test of saliva-swapping vs water and see to what degree this affects self-reported measures. Or if that's too hard to get past IRB, have one towel that's been touched by 50 people and one towel that's completely clean, and see if touching that has any subjective effects.
It feels like the kind of things you could test on 1,000 students in a weekend for free.