Q:

single word counterpart to the word 'insidious' when the results are beneficial

I’ve been trying to think of a single word counterpart to the word insidious.

Oxford Dictionaries:
  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?

A:

Well, it's kind of simplistic, but it can't be construed as negative...  You could use "blossoming". 
If you're talking about a societal trend or social movement, I'm afraid the most recognized term is a phrase: grass-roots.  (It usually has positive connotations.) (US)

A:

The term burgeoning can mean

To begin to grow or blossom. American Heritage

However, it often has a connotation of rapid or increasing growth.

A:

Perhaps "nurturing" would be suitable.

