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, carriers, they are brain enhancers, and in others a fatal brain plague, a living addiction. This is the story of one woman's psychological and moral struggle to adjust to having an ambitious colony of microbes living permanently in her own head. This novel is one of the most powerful and involving SF novels of the year.