The virus ripped, or "unglued" DNA strands. The strand would attempt to repair itself, but its proteins were scrambled in the attempt. Death was the end result for the infected host, either resulting from the immune system's rejection of or the physical ripping apart of the body. It changed human DNA, causing flesh to fall off, or extra limbs to grow onto parts of the body where there shouldn't be limbs.
Thousands of years before humans arrived on Lusitania, the Descoladores created the virus and spread it in a wide swath throughout the galaxy. Eventually, a ship came to Lusitania, home of the Pequeninos, and infected the ecosystem of the planet.
Most Lusitanian species died out because they could not adapt to the virus, but several incorporated the Descolada into their life cycles and became immune to its devastating effects, including the intelligent Pequeninos.
Speaker for the Dead
Millennia later, humans originating from the planet Baía settled on Lusitania. They quickly discovered the Descolada, and named it appropriately. As soon as the symptoms of the virus were discovered, research for a cure began. Vast amounts of the colonists died off before Gusto and Cida von Hesse could design a temporary cure for the disease.
After Andrew Wiggin arrived on the planet, the Xenobiologists learned more about the Descolada and how it interacted with the native life of the planet. The virus had paired each species with another, one becoming the other when they died.