> I'd say no pathogens but they'll come with us regardless.

I think NOT bringing the "right" microbes would be the problem, since it's not well understood. Childhood leukemia is linked to over cleanliness (lack of hepatitis A) [1]. Fecal transplants are being tested for neurological disorders, with some positive results!

Similarly, germ free rats have all sorts of problems, from under developed intestines to significant problems with brain development!

Who knows what's required, that we just happen to pick up while kids, sharing snot and mud with each other.

[1] https://pubmed.ncbi.nlm.nih.gov/9684709/