And when your two year old reaches for dog poop, do you explain the germ theory of disease to her, or slap her hand and say "no"? Assuming G-d did explain the germ theory of disease, how would these saved billions of people feed themselves? You'd have to learn the entirety of modern civilization in one fell swoop.
You are saying that the people who built Rome were too dumb to understand basic germ theory?
God is supposed to have unlimited power and ability. Had he simply explained to people during the time of Jesus that germs exist and people should wash their hands before eating, it would have saved billions. Had Europe not gone through the plague which was caused by having open sewers in the street, millions would have been saved.
I do not think that argument you put forth holds any water. People were smart enough to do some amazing things back in the day. I think by the time Jesus came it would have been easy enough to explain germs even at a basic level.