Gargling with plain water 3x/day reduced upper respiratory infections by ~36% in a randomized trial
(pubmed.ncbi.nlm.nih.gov)
When I come home from being among people, I blow my nose and wash my hands + face, and I feel like that helps, too. I mean, logically it should help, since I'm physically removing bacteria and whatnot.
But yeah, I don't know, feels a bit weird that we're only telling people to wash their hands, when airborn bacteria/viruses are a thing...