Howard V. Hendrix, Empty Cities of the Full Moon (Ace, 2001)
Empty Cities of the Full Moon blends hard science fiction with a
thoughtful and illuminating tale that grapples with the question
of what it means to be human.  A third of the way into the 21st
century, a pandemic rages, wiping out swathes of human
civilizations and transforming many of the survivors. The plague
manifests as a driving urge to drum and dance, more so during
full moons, so initially it is perceived as a widespread fad.
But when the drummers fall into comatose states marked by
intensive dreaming, stigmata and shape-shifting, the population
realizes that it is in deep trouble and that the world as they
knew it is gone forever.  Three decades later, the planet is
sparsely populated with Trufolk, the shape shifting Wer folk and
the biologically engineered Merfolk who guard and defend the New
Bahamian Polity founded and governed by erstwhile billionaire
Cameron Spires. Spires set up this refuge when the world went
mad, and now the residen