Wizard is the middle volume in John Varley’s near-future Gaea Trilogy. In stark contrast to industry practice, the trilogy consists of exactly three novels.
Chris Minor and Robin the Nine-Fingered both have genetic diseases that, while quite different, may have the same cure. All the Earthman and spacewoman need do is appeal to God and pray for mercy. Or rather, appeal to a god: Gaea, a 1,300-km diameter living, intelligent torus orbiting Saturn. Having complete command of matters biological — at least within the torus — altering genes is a trivial task for the ancient being.
Convincing Gaea to perform a cure will be trickier.