There are microorganisms which do mind control on humans, famously Toxoplasma. In the near future, I predict we will discover many more of them, catalogue them, measure their prevalence, and eliminate them using public health techniques.
Humanity might then see a new age in which a vast mental fog has gotten lifted.
But what if it was a microorganism infection in the population that helped someone achieve or maintain power? Would they be keen to deploy public health techniques to eliminate that microorganism?
What if it is a microorganism that is helping prevent anarchy? Perhaps encouraging people to be docile, or not to fight each other, or form social connections evolutionarily advantageous for the microorganism to propagate?