Gross

I have been thinking about what gross means for some time now.
My little nephew freaked out not long ago when he asked for a napkin at lunch and I handed him one that had a tiny bit of food on it. He couldn't handle even sitting near it. It was too gross.
And I was like "You'll live."
When I was in kindergarten I remember finding a stick of gum on the bus, still in the wrapper. I chewed it, when I got home my Mom asked me where I got she was not impressed.
To children somethings seem beyond disgusting that parents are like "yeah, I deal with that all the time." But then other things that seem totally fine to kids, completely gross their parents out.
How do people learn what is "gross" and what isn't.
I see the same things in adults. I watched this video not long ago. The little boy smearing his dad with spaghetti sauce totally grossed me out. (Which by the way is NOT the point of the video). And yet most days of the dig I smeared my own face and arms with dirt and vanilla (the vanilla was to keep the bugs away, the dirt just happened). And that didn't bother me at all.

Honestly, I think food grosses me out more than a lot of other things. Like cleaning up food, and leftover food and such, not eating it (usually). But other people that came to the dig didn't even like to touch the dirt without a glove and told their children not to touch worms.
I feel like sometimes in our world we act like things are either gross or they aren't. That they are just inherently that way. At least that is how we talk to children. "That's gross, don't touch/eat it." Not "Please don't touch that. It is unsanitary/dirty/etc." So how do we, as individuals, learn what gross is?