I have no strong desire to do this, I'm playing a Warlord. My question is, in all the thousands of years of magical civilization, why hasn't anyone else? There is nothing in the rules of logic, magic, or the game that specifically disallows it helpful bacteria. And there is magically harmful bacteria in the game. Why hasn't some NPC cleric lost someone to some evil plague and created a good plague to fight it? Can it not be done? Then we need to have a pretty extensive explanation as to why bacteria helpful to humanoids is immune to magic.
Mechanics are not the be all and end all of story. Personalities are the meat of story, whether it's a magician that would be helpful to the world at large by creating such a disease, or the very malevolent Lord of Plagues who successfully twists such work.
The answer to your question for any given world generally boils to the following.
1. No one has thought to try it.. when it comes to magic, magicians typically think of power or war, because that's the way they are oriented to think.
2. It's been tried, and either failed or the results were like that of Miri's World.... or worse.
These questions are worth exploring... but they need to be looked at beyond the tunnel vision of just taking mechanics as the determining factor. Also keep in mind... Truly Perfect Worlds don't really have much story potential, as Larry Niven's Teela Brown World short story amply demonstrates.