I’ve been trying to think of a single word counterpart to the word insidious.
insidious (adjective), Proceeding in a gradual, subtle way, but with very harmful effects.
The meaning I require (to describe the action of yeast say within dough or a must, and by metaphorical extension, small acts of kindness in society) is
Proceeding in a gradual, subtle way, with beneficial effects that eventually make a great difference.
There are related words with no implications of good or evil effect: Thesaurus.com gives synonyms for pervasive including permeating and pervading.
Is there though a single word (not a phrase, proverb … please) with the required meaning?