Agypestis

An intelligent microbe race that can live symbiotically in other intelligent beings is colonizing the human race throughout the civilized universe. And each colony of microbes has its own personality, good or bad. In some people the microbes are brain enhancers, but in others they cause a fatal brain plague, a living addiction. Becoming a human carrier, then, is a threatening, potentially deadly fate.

This is the story of Chrysoberyl: one woman’s psychological and moral struggle to adjust to having an ambitious colony of microbes living permanently in her own head. Furthermore, the colonists can talk to her and she to them, producing a kind of internal drama that counterpoints the external action.