Crucible by Nancy Kress
Jake Holman, a man trying to escape a dark past, brought together a diverse group of thousands to settle on a new world. The humans found themselves caught in the crossfire of a galaxy-spanning war between two disparate species: aggressive, militaristic humanoids known as Furs, and passive, plant like creatures known as Vines. Having now cast their lot with the peaceful Vines, humanity faces all-out war against the technologically superior Furs. Their only hope? A virus designed by the Vines to remove all aggressiveness from the Furs. Can it spread fast enough to save not only Holman's colony, but the rest of humanity?